Counter cache Rails guides fix

You need to specify the `counter_cache` option on the `has_many` side of the association when using a custom counter cache column. This option is documented on the `has_many` association here: http://apidock.com/rails/ActiveRecord/Associations/ClassMethods/has_many

```
:counter_cache
    This option can be used to configure a custom named :counter_cache. You only need this 
    option, when you customized the name of your :counter_cache on the belongs_to association.
```