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

This line is required for app manifests, so it should be in the sample
manifest in the tutorial.

BUG=374672
NOTRY=true

Review URL: https://codereview.chromium.org/293523003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@271513 0039d316-1c4b-4281-b951-d872f2087c98

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

@@ -47,6 +47,7 @@ describes this manifest in detail):
   "name": "Hello World!",
   "description": "My first Chrome App.",
   "version": "0.1",
+  "manifest_version": 2,
   "app": {
     "background": {
       "scripts": ["background.js"]