Tiny code example fix [ci skip]

The code example wasn't properly formatted because of missing line
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guides/source/action_cable_overview.md

@@ -123,8 +123,9 @@ or unauthorized).
 If your authentication approach includes using a session, you use cookie store for the
 session, your session cookie is named `_session` and the user ID key is `user_id` you
 can use this approach:
+
 ```ruby
-  verified_user = User.find_by(id: cookies.encrypted['_session']['user_id'])
+verified_user = User.find_by(id: cookies.encrypted['_session']['user_id'])
 ```
 
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