Fix typo in new background page syntax ("page" takes a string, not an array)

chrome/common/extensions/docs/static/background_pages.html

 <div id="pageData-name" class="pageData">Background Pages</div>
 <div id="pageData-showTOC" class="pageData">true</div>
 
 <p>
 A common need for extensions is to have
 a single long-running script to manage some task or state.
 Background pages to the rescue.
 </p>
 
 <p>
@@ skipping 47 matching lines @@
 If you need to specify HTML
 in your background page, you can
 do that using the <code>page</code>
 property instead:
 </p>
 
 <pre>{
   "name": "My extension",
   ...
   <b>"background": {
-    "page": ["background.html"]
+    "page": "background.html"
   }</b>,
   ...
 }</pre>
 
 <p>
 If you need the browser to start up early&mdash;so
 you can display notifications, for example&mdash;then
 you might also want to specify the
 <a href="manifest.html#permissions">"background" permission</a>.
 </p>
@@ skipping 67 matching lines @@
   &lt;body>
     &lt;p>
     Image here:
     &lt;/p>
 
     &lt;img id="target" src="white.png" width="640" height="480">
 
   &lt;/body>
 &lt;/html>
 </pre>