Fixed CollectionView doc reference to former OnItemAdded event callback

This commit changed the 'item:added' event to 'before:item:added' and 'after:item:added'. 

https://github.com/marionettejs/backbone.marionette/commit/fade8d18d07efa5ac5d32a938f8fd857fedeb06f#src/marionette.collectionview.js

The docs describe an OnItemAdded event callback. I've updated the docs to represent the current OnBeforeItemAdded and OnAfterItemAdded event callbacks instead.