Make state immutable in tutorial (eek)

Author: petehunt

docs/docs/tutorial.md

@@ -629,8 +629,8 @@ var CommentBox = React.createClass({
   },
   handleCommentSubmit: function(comment) {
     var comments = this.state.data;
-    comments.push(comment);
-    this.setState({data: comments});
+    var newComments = comments.concat([comment]);
+    this.setState({data: newComments});
     $.ajax({
       url: this.props.url,
       type: 'POST',