Update references to sandbox app samples.

chrome/common/extensions/docs/server2/templates/articles/app_external.html

 <h1>Embed Content</h1>
 
 
 <p>
 The <a href="app_architecture.html#security">packaged apps security model</a> disallows
 external content in iframes and
 the use of inline scripting and <code>eval()</code>.
 You can override these restrictions,
 but your external content must be isolated from the app.
 </p>
 
 <p>
 Isolated content cannot directly
 access the app's data or any of the APIs.
 Use cross-origin XMLHttpRequests
 and post-messaging to communicate between the event page and sandboxed content
 and indirectly access the APIs.
 </p>
 
 <p class="note">
-<b>API Samples: </b>
+<b>API Sample: </b>
 Want to play with the code?
 Check out the
-<a href="https://github.com/GoogleChrome/chrome-app-samples/tree/master/eval-in-iframe">eval-in-iframe</a>
-and <a href="https://github.com/GoogleChrome/chrome-app-samples/tree/master/regex-tester">regex-tester</a> samples.
+<a href="https://github.com/GoogleChrome/chrome-app-samples/tree/master/sandbox">sandbox</a> sample.
 </p>
 
 <h2 id="external">Referencing external resources</h2>
 
 <p>
 The <a href="app_csp.html">Content Security Policy</a> used by apps disallows
 the use of many kinds of remote URLs, so you can't directly reference external
 images, stylesheets, or fonts from an app page. Instead, you can use use
 cross-origin XMLHttpRequests to fetch these resources,
 and then serve them via <code>blob:</code> URLs.
@@ skipping 236 matching lines @@
 </p>
 
 <pre>
 var messageHandler = function(e) {
   console.log('Background script says hello.', e.data);
 };
 
 window.addEventListener('message', messageHandler);
 </pre>
 
-<p class="backtotop"><a href="#top">Back to top</a></p>
+<p class="backtotop"><a href="#top">Back to top</a></p>