Documentation: First App: Added manifest_version line to example code.

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

 <meta name="doc-family" content="apps">
 <h1>Create Your First App</h1>
 
 
 <p>
 This tutorial walks you through creating your first Chrome App.
 Chrome Apps are structured similarly to extensions
 so current developers will recognize the manifest and packaging methods.
 When you're done,
 you'll just need to produce a zip file of your code and assets
@@ skipping 29 matching lines @@
 First create your <code>manifest.json</code> file
 (<a href="manifest.html">Formats: Manifest Files</a>
 describes this manifest in detail):
 </p>
 
 <pre data-filename="manifest.json">
 {
   "name": "Hello World!",
   "description": "My first Chrome App.",
   "version": "0.1",
+  "manifest_version": 2,
   "app": {
     "background": {
       "scripts": ["background.js"]
     }
   },
   "icons": { "16": "calculator-16.png", "128": "calculator-128.png" }
 }
 </pre>
 
 <p class="note">
@@ skipping 118 matching lines @@
   </li>
   <li>
     <code>--app-id=ajjhbohkjpincjgiieeomimlgnll</code> launches an app
     already loaded into Chrome. It does not restart any previously running
     app, but it does launch the new app with any updated content.
   </li>
 </ul>
 </p>
 
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