[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/exception b/include/exception
index 5f54861..886daac 100644
--- a/include/exception
+++ b/include/exception
@@ -328,6 +328,6 @@
 {
 }
 
-}  // std
+} // namespace std
 
 #endif // _LIBCPP_EXCEPTION