Remove .html extension from links

chrome/common/extensions/docs/templates/articles/app_codelab3_mvc.html

 <h1 id="lab_3_model_view_controller">Create MVC</h1>
 
 <p>Whenever your application grows beyond a single script with a few dozen lines,
 it gets harder and harder to manage without a good separation
 of roles among app components.
 One of the most common models for structuring a complex application,
 no matter what language,
 is the Model-View-Controller (MVC) and its variants,
 like Model-View-Presentation (MVP).</p>
 
 <p>There are several frameworks to help apply
-<a href="app_frameworks.html">MVC concepts</a>
+<a href="app_frameworks">MVC concepts</a>
 to a Javascript application, and most of them,
 as long as they are CSP compliant, can be used in a Chrome App.
 In this lab,
 we'll add an MVC model using both pure JavaScript and
 the <a href="http://angularjs.org/">AngularJS</a> framework.
 Most of the AngularJS code from this section was copied,
 with small changes, from the AngularJS Todo tutorial.</p>
 
 <p class="note"><b>Note:</b>
 Chrome Apps don&#39;t enforce any specific framework or programming style.
@@ skipping 375 matching lines @@
 <h3 id="check2">Check the results</h3>
 
 <p>
 Check the results by reloading the app: open the app, right-click and select Reload App.</li>
 </p>
 
 <h2 id="takeaways_">Takeaways</h2>
 
 <ul>
 <li><p>Chrome Apps are
-<a href="offline_apps.html">offline first</a>,
+<a href="offline_apps">offline first</a>,
 so the recommended way to include third-party scripts is to download
 and package them inside your app.</p></li>
 <li><p>You can use any framework you want,
 as long as it complies with Content Security Policies
 and other restrictions that Chrome Apps are enforced to follow.</p></li>
 <li><p>MVC frameworks make your life easier.
 Use them, specially if you want to build a non-trivial application.</p></li>
 </ul>
 
 <h2 id="you_should_also_read">You should also read</h2>
 
 <ul>
-<li><p><a href="angular_framework.html">Build Apps with AngularJS</a> tutorial</p></li>
+<li><p><a href="angular_framework">Build Apps with AngularJS</a> tutorial</p></li>
 <li><p><a href="http://angularjs.org/">AngularJS Todo</a> tutorial</p></li>
 </ul>
 
 <h2 id="what_39_s_next_">What's next?</h2>
 
-<p>In <a href="app_codelab5_data.html">4 - Save and Fetch Data</a>,
+<p>In <a href="app_codelab5_data">4 - Save and Fetch Data</a>,
 you will modify your Todo list app so that Todo items are saved.</p>