[DOC beta] Remove Ember.ArrayController from initializer example

`Ember.ArrayController` is deprecated since Ember 1.11.
ref: http://emberjs.com/deprecations/v1.x/#toc_arraycontroller

app/initializers/ember-data.js

@@ -19,7 +19,7 @@ import 'ember-data/-private/core';
     adapter: 'custom'
   });
 
-  App.PostsController = Ember.ArrayController.extend({
+  App.PostsController = Ember.Controller.extend({
     // ...
   });