[libc++][NFC] Update namespace comments in include/

update the namspace comments in include/

Reviewed By: ldionne, #libc

Spies: smeenai, libcxx-commits

Differential Revision: https://reviews.llvm.org/D114947

NOKEYCHECK=True
GitOrigin-RevId: d2b0df35afb7184f5a68f67d6ed0c6230688df7f
diff --git a/include/regex b/include/regex
index 815ff7d..8203c81 100644
--- a/include/regex
+++ b/include/regex
@@ -978,7 +978,7 @@
     __re_err_parse
 };
 
-}  // regex_constants
+} // namespace regex_constants
 
 class _LIBCPP_EXCEPTION_ABI regex_error
     : public runtime_error