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 ~~AssociationProxy~~ => Association.
    #
    #   CollectionAssociation:
    #     HasManyAssociation => has_many
    #       HasManyThroughAssociation + ThroughAssociation => has_many :through
```

to match this change from the release notes

```ruby
* `ActiveRecord::Associations::AssociationProxy` has been split. There is now an 
`Association` class (and subclasses) which are responsible for operating on associations, 
and then a separate, thin wrapper called `CollectionProxy`, which proxies collection 
associations. This prevents namespace pollution, separates concerns, and will allow further 
refactorings.
```