[libc++][NFC] Fix slightly incorrect comment in PSTL documentation

Author: ldionne

libcxx/include/__algorithm/pstl_backend.h

@@ -41,13 +41,13 @@ A PSTL parallel backend is a tag type to which the following functions are assoc
             class _ForwardIterator2,
             class _ForwardOutIterator,
             class _Comp>
-  _ForwardOutIterator __pstl_merge(_Backend,
-                                   _ForwardIterator1 __first1,
-                                   _ForwardIterator1 __last1,
-                                   _ForwardIterator2 __first2,
-                                   _ForwardIterator2 __last2,
-                                   _ForwardOutIterator __result,
-                                   _Comp __comp);
+  optional<_ForwardOutIterator> __pstl_merge(_Backend,
+                                             _ForwardIterator1 __first1,
+                                             _ForwardIterator1 __last1,
+                                             _ForwardIterator2 __first2,
+                                             _ForwardIterator2 __last2,
+                                             _ForwardOutIterator __result,
+                                             _Comp __comp);
 
   template <class _ExecutionPolicy, class _InIterator, class _OutIterator, class _UnaryOperation>
   optional<_OutIterator>