Fix mistaken guide how to extend association proxy with module

There is no method `extending` for `Module`. It should be only `include` here. Tested by me both cases `exdend` and `include`. Only `include` works.

This is my code which I used to test code:

```ruby
module AssociationExtensions
  module UniqMetaIds
    def uniq_meta_ids
      select(:meta_id).uniq.pluck(:meta_id)
    end
  end
end

class MetaService < ActiveRecord::Base
  has_many :meta_settings do
    include AssociationExtensions::UniqMetaIds
  end
end

describe MetaService do
  describe '#meta_settings' do
    it { expect(subject.meta_settings.uniq_meta_ids).to be_blank }
  end
end
```