Fix CollectionView docs to use extend rather than create

https://github.com/emberjs/ember.js/blob/v1.0.0-rc.6.1/packages/ember-views/lib/views/collection_view.js#L106 specifies to use `extend`, and in my brief attempt to use this class, using `create` yields
```
Uncaught TypeError: Object [object Object] has no method 'create' 
```