Fix a missing update that C compiles default to gnu17.

[srhines]

Summary

https://reviews.llvm.org/D75383 switched the C default to gnu17, but
missed this instance.

[nickdesaulniers]

bah!

$ find . -name \*.rst | xargs grep -n gnu11
./clang/docs/UsersManual.rst:2462:The supported modes for C are c89, gnu89, c99, gnu99, c11, gnu11, c17, gnu17,
./clang/docs/CommandGuide/clang.rst:136:  | ``gnu11``
./clang/docs/CommandGuide/clang.rst:149: The default C language standard is ``gnu11``, except on PS4, where it is
./clang/docs/ReleaseNotes.rst:165:  upgraded from gnu11 to gnu17.
./clang/docs/LanguageExtensions.rst:1019:features are enabled with the ``-std=c11`` or ``-std=gnu11`` option when

Thanks for cleaning up my mess. The rest of these instance look fine to me.

clang/docs/CommandGuide/clang.rst

Supported values for the C language are:
   | ``iso9899:2017``
 
    ISO C 2017
 
   | ``gnu17``
 
    ISO C 2017 with GNU extensions
 
- The default C language standard is ``gnu11``, except on PS4, where it is
+ The default C language standard is ``gnu17``, except on PS4, where it is
  ``gnu99``.
 
  Supported values for the C++ language are:
 
   | ``c++98``
   | ``c++03``
 
    ISO C++ 1998 with amendments