Update constant_autoloading_and_reloading.md

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Follow-up to previous commit

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Fix previous merge

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You need to specify the counter_cache option on the has_many side of the association when using a custom counter cache column. This is documented on the has_many association here: http://apidock.com/rails/ActiveRecord/Associations/ClassMethods/has_many [ci skip]
Merge pull request #18046 from stephenminded/counter_cache_docs_fix

Counter cache Rails guides fix
Merge branch 'patch-8' of https://github.com/jonatack/rails into jonatack-patch-8
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Conflicts:
	guides/source/constant_autoloading_and_reloading.md
Merge branch 'jonatack-patch-8' [ci skip]