Minor fixes to Type ODS docs.

[mehdi_amini]

(Feel free to push such trivial change as post-commit review)

mlir/docs/OpDefinitions.md

def IntegerType : Test_Type<"TestInteger"> {
   let parser = [{
     if (parser.parseLess())
       return Type();
     int width;
     if ($_parser.parseInteger(width))
       return Type();
     if ($_parser.parseGreater())
       return Type();
-    return get(ctxt, width);
+    return get($_ctxt, width);
   }];
+}
 ```
 
 ### Type name
 
 The name of the C++ class which gets generated defaults to
 `<classParamName>Type` (e.g. `TestIntegerType` in the above example). This can
 be overridden via the `cppClassName` field. The field `mnemonic` is to specify
 the asm name for parsing. It is optional and not specifying it will imply that