[libc++] Fix typo in comment at __optional_storage_base

Small typo fix(es) for struct definition of __optional_storage_base for
a reference type.

Reviewed By: #libc, jloser, philnik

Differential Revision: https://reviews.llvm.org/D126621

libcxx/include/optional

@@ -393,9 +393,9 @@ struct __optional_storage_base : __optional_destruct_base<_Tp>
     }
 };
 
-// optional<T&> is currently required ill-formed, however it may to be in the
-// future. For this reason it has already been implemented to ensure we can
-// make the change in an ABI compatible manner.
+// optional<T&> is currently required to be ill-formed. However, it may
+// be allowed in the future. For this reason, it has already been implemented
+// to ensure we can make the change in an ABI-compatible manner.
 template <class _Tp>
 struct __optional_storage_base<_Tp, true>
 {