[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/stdexcept b/include/stdexcept
index ddbc630..c1dc056 100644
--- a/include/stdexcept
+++ b/include/stdexcept
@@ -209,7 +209,7 @@
 #endif
 };
 
-}  // std
+} // namespace std
 
 _LIBCPP_BEGIN_NAMESPACE_STD