[clang-format][docs] Fix incorrect 'clang-format 13' configuration ...

...options markers

Note: Option 'IndentRequiresClause' was previously known as
'IndentRequires' but the version marker should still indicate
'clang-format 15' as this option most recent name wasn't accessible
earlier and it would produce:
error: unknown key 'IndentRequiresClause'

Differential Revision: https://reviews.llvm.org/D119682