[FormatVariadic] Mark index as required in docstring

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.

Differential Revision: https://reviews.llvm.org/D118833

Author: kastiglione

llvm/include/llvm/Support/FormatVariadic.h

@@ -172,7 +172,7 @@ template <typename Tuple> class formatv_object : public formatv_object_base {
 // 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 "}">