Use separate code blocks for separate files [ci-skip]

Follow-up to rails#50342.

Author: jonathanhefner

guides/source/association_basics.md

@@ -804,6 +804,9 @@ module MyApplication
     end
   end
 end
+```
+
+```ruby
 # app/models/my_application/business/account.rb
 module MyApplication
   module Business