[FormatVariadic] Mark index as required in docstring (NFC)

[kastiglione]

Summary

After looking at the formatv docstring, I thought the index was optional (as it
is in other languages). This changes the header docs to show index instead of
[index], to indicate that it is required.

llvm/include/llvm/Support/FormatVariadic.h

 
 // Format text given a format string and replacement parameters.
 //
 // ===General Description===
 //
 // Formats textual output.  `Fmt` is a string consisting of one or more
 // replacement sequences with the following grammar:
 //
-// rep_field ::= "{" [index] ["," layout] [":" format] "}"
+// rep_field ::= "{" index ["," layout] [":" format] "}"
 // index     ::= <non-negative integer>
 // layout    ::= [[[char]loc]width]
 // format    ::= <any string not containing "{" or "}">
 // char      ::= <any character except "{" or "}">
 // loc       ::= "-" | "=" | "+"
 // width     ::= <positive integer>
 //
 // index   - A non-negative integer specifying the index of the item in the