fix CollectionAssociation's comments

change CollectionAssociation's comments to say Association instead of AssociationProxy to match changes for 3.1 removing Association proxy.

```ruby
    # CollectionAssociation is an abstract class that provides common stuff to
    # ease the implementation of association proxies that represent
    # collections. See the class hierarchy in Association ~~AssociationProxy~~.
    #
    #   CollectionAssociation:
    #     HasManyAssociation => has_many
    #       HasManyThroughAssociation + ThroughAssociation => has_many :through
```