Apply LWG 2017.  This is a only a documentation change.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@191140 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/functional b/include/functional
index fdb8e0f..c3c713e 100644
--- a/include/functional
+++ b/include/functional
@@ -56,7 +56,7 @@
 
     // invoke
     template <class... ArgTypes>
-      typename result_of<T(ArgTypes...)>::type
+      typename result_of<T&(ArgTypes&&...)>::type
           operator() (ArgTypes&&...) const;
 };