doc(iterator): fix 示例代码

docs/iterator.md

@@ -181,7 +181,7 @@ Obj.prototype[Symbol.iterator] = function(){
   function next(){
     if (current){
       var value = current.value;
-      var done = current.next === undefined;
+      var done = current.next === null;
       current = current.next;
       return {
         done: done,