Fix japanese tutorial

docs/docs/tutorial.ja-JP.md

@@ -324,7 +324,7 @@ var CommentList = React.createClass({
   render: function() {
     var commentNodes = this.props.data.map(function (comment) {
       return (
-        <Comment author={comment.author}>
+        <Comment author={comment.author} key={comment.id}>
           {comment.text}
         </Comment>
       );