fix 'feeback' typo in stock app in menus

sky/examples/stocks/lib/stock_menu.dart

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 import 'package:sky/framework/components/popup_menu.dart';
 import 'package:sky/framework/fn.dart';
 import 'package:sky/framework/theme/view-configuration.dart';
 
 class StockMenu extends Component {
   static final Style _style = new Style('''
     position: absolute;
     right: 8px;
     top: ${8 + kStatusBarHeight}px;''');
 
   PopupMenuController controller;
 
   StockMenu({Object key, this.controller}) : super(key: key);
 
   UINode build() {
     return new StyleNode(
       new PopupMenu(
         controller: controller,
         items: [
           [new Text('Add stock')],
           [new Text('Remove stock')],
-          [new Text('Help & feeback')],
+          [new Text('Help & feedback')],
         ],
         level: 4),
         _style
     );
   }
 }