minor documentation update

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@132286 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/functional b/include/functional
index 82a4a92..2ea1103 100644
--- a/include/functional
+++ b/include/functional
@@ -384,7 +384,7 @@
 
     function& operator=(const function&);
     function& operator=(function&&) noexcept;
-    function& operator=(nullptr_t);
+    function& operator=(nullptr_t) noexcept;
     template<class F>
       function& operator=(F&&);
     template<class F>