[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/string b/include/string
index 313a7b5..ed2fb7e 100644
--- a/include/string
+++ b/include/string
@@ -4571,8 +4571,8 @@
     {
         return basic_string<char32_t> (__str, __len);
     }
-  }
-}
+  } // namespace string_literals
+} // namespace literals
 #endif
 
 _LIBCPP_END_NAMESPACE_STD