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

R=aa@chromium.org

Review URL: https://chromiumcodereview.appspot.com/9582009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@124680 0039d316-1c4b-4281-b951-d872f2087c98

chrome/common/extensions/docs/background_pages.html

@@ -265,7 +265,7 @@ property instead:
   "name": "My extension",
   ...
   <b>"background": {
-    "page": ["background.html"]
+    "page": "background.html"
   }</b>,
   ...
 }</pre>

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

@@ -65,7 +65,7 @@ property instead:
   "name": "My extension",
   ...
   <b>"background": {
-    "page": ["background.html"]
+    "page": "background.html"
   }</b>,
   ...
 }</pre>