[libc++][NFC] Fix some standard-mandated includes comments

Reviewed By: ldionne, #libc

Spies: libcxx-commits

Differential Revision: https://reviews.llvm.org/D134447

NOKEYCHECK=True
GitOrigin-RevId: 473a1605063421a36b82b31dc5a7e9533d6e229e
diff --git a/include/valarray b/include/valarray
index e0f0753..f28e471 100644
--- a/include/valarray
+++ b/include/valarray
@@ -361,6 +361,8 @@
 #include <version>
 
 // standard-mandated includes
+
+// [valarray.syn]
 #include <initializer_list>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)