[libc++] Fix some one-off typos in comments. NFCI.

GitOrigin-RevId: b6f191741562ab32a3e210c8661df09b546275cc
diff --git a/include/optional b/include/optional
index 6cd5148..570fa11 100644
--- a/include/optional
+++ b/include/optional
@@ -657,7 +657,7 @@
       }
       template <class _Up, class _QUp = _QualUp>
       static constexpr bool __enable_assign() {
-          // Construction and assignability of _Qup to _Tp has already been
+          // Construction and assignability of _QUp to _Tp has already been
           // checked.
           return !__check_constructible_from_opt<_Up>::value &&
               !__check_assignable_from_opt<_Up>::value;