[mlir][doc] Fix the EBNF description of mlir syntax in language reference doc

[zzz00yx]

Summary

According to the EBNF syntax described in the 'Common syntax' chapter, literal characters should be surrounded by backticks (`). However, in some sections of this document, single quotes (') are used instead. So, fix them.

[zzz00yx]

At line 705 and line 837, the single quotes are removed but not be replaced.

[zzz00yx]

BTW, if this patch Looks Good To You, could you please help me land it to the repo, because I have no commit access.
My information: Yan Xin <[e-mail]>

[zzz00yx]

Thanks for your review, I have no commit access, could you help me land it to the repo please?

mlir/docs/LangRef.md

 ssa-use-and-type-list ::= ssa-use-and-type (`,` ssa-use-and-type)*
 
 function-type ::= (type | type-list-parens) `->` (type | type-list-parens)
 ```
 
 ### Type Aliases
 
 ```
-type-alias-def ::= '!' alias-name '=' type
-type-alias ::= '!' alias-name
+type-alias-def ::= `!` alias-name `=` type
+type-alias ::= `!` alias-name
 ```
 
 MLIR supports defining named aliases for types. A type alias is an identifier
 that can be used in the place of the type that it defines. These aliases *must*
 be defined before their uses. Alias names may not contain a '.', since those
 names are reserved for [dialect types](#dialect-types).
 
 Example:

 ### Dialect Types
 
 Similarly to operations, dialects may define custom extensions to the type
 system.
 
 ```
 dialect-namespace ::= bare-id
 
-dialect-type ::= '!' (opaque-dialect-type | pretty-dialect-type)
+dialect-type ::= `!` (opaque-dialect-type | pretty-dialect-type)
 opaque-dialect-type ::= dialect-namespace dialect-type-body
-pretty-dialect-type ::= dialect-namespace '.' pretty-dialect-type-lead-ident
+pretty-dialect-type ::= dialect-namespace `.` pretty-dialect-type-lead-ident
                                               dialect-type-body?
-pretty-dialect-type-lead-ident ::= '[A-Za-z][A-Za-z0-9._]*'
+pretty-dialect-type-lead-ident ::= `[A-Za-z][A-Za-z0-9._]*`
 
-dialect-type-body ::= '<' dialect-type-contents+ '>'
+dialect-type-body ::= `<` dialect-type-contents+ `>`
 dialect-type-contents ::= dialect-type-body
-                            | '(' dialect-type-contents+ ')'
-                            | '[' dialect-type-contents+ ']'
-                            | '{' dialect-type-contents+ '}'
-                            | '[^\[<({\]>)}\0]+'
+                            | `(` dialect-type-contents+ `)`
+                            | `[` dialect-type-contents+ `]`
+                            | `{` dialect-type-contents+ `}`
+                            | [^\[<({\]>)}\0]+
 ```
 
 Dialect types are generally specified in an opaque form, where the contents
 of the type are defined within a body wrapped with the dialect namespace
 and `<>`. Consider the following examples:
 
 ```mlir
 // A tensorflow string type.

 
 When properties are adopted, only discardable attributes are stored in the
 top-level dictionary, while inherent attributes are stored in the properties
 storage.
 
 ### Attribute Value Aliases
 
 ```
-attribute-alias-def ::= '#' alias-name '=' attribute-value
-attribute-alias ::= '#' alias-name
+attribute-alias-def ::= `#` alias-name `=` attribute-value
+attribute-alias ::= `#` alias-name
 ```
 
 MLIR supports defining named aliases for attribute values. An attribute alias is
 an identifier that can be used in the place of the attribute that it defines.
 These aliases *must* be defined before their uses. Alias names may not contain a
 '.', since those names are reserved for
 [dialect attributes](#dialect-attribute-values).
 

 
 ### Dialect Attribute Values
 
 Similarly to operations, dialects may define custom attribute values.
 
 ```
 dialect-namespace ::= bare-id
 
-dialect-attribute ::= '#' (opaque-dialect-attribute | pretty-dialect-attribute)
+dialect-attribute ::= `#` (opaque-dialect-attribute | pretty-dialect-attribute)
 opaque-dialect-attribute ::= dialect-namespace dialect-attribute-body
-pretty-dialect-attribute ::= dialect-namespace '.' pretty-dialect-attribute-lead-ident
+pretty-dialect-attribute ::= dialect-namespace `.` pretty-dialect-attribute-lead-ident
                                               dialect-attribute-body?
-pretty-dialect-attribute-lead-ident ::= '[A-Za-z][A-Za-z0-9._]*'
+pretty-dialect-attribute-lead-ident ::= `[A-Za-z][A-Za-z0-9._]*`
 
-dialect-attribute-body ::= '<' dialect-attribute-contents+ '>'
+dialect-attribute-body ::= `<` dialect-attribute-contents+ `>`
 dialect-attribute-contents ::= dialect-attribute-body
-                            | '(' dialect-attribute-contents+ ')'
-                            | '[' dialect-attribute-contents+ ']'
-                            | '{' dialect-attribute-contents+ '}'
-                            | '[^\[<({\]>)}\0]+'
+                            | `(` dialect-attribute-contents+ `)`
+                            | `[` dialect-attribute-contents+ `]`
+                            | `{` dialect-attribute-contents+ `}`
+                            | [^\[<({\]>)}\0]+
 ```
 
 Dialect attributes are generally specified in an opaque form, where the contents
 of the attribute are defined within a body wrapped with the dialect namespace
 and `<>`. Consider the following examples:
 
 ```mlir
 // A string attribute.