tutorial step-2: missing slash on closing div tag

http://www.polymer-project.org/docs/start/tutorial/step-2.html

``` html
...   
  </core-toolbar>
    <div class="container" layout vertical center>
      <post-card>
      <img width="70" height="70" 
        src="../images/avatar-07.svg">
        <h2>Another Developer</h2>
        <h3>I'm composing with shadow DOM!</h3>
      </post-card>
    <div>
  ...
```
