Fix formatting in Action Cable guide [ci skip]

Author: prathamesh-sonpatki

guides/source/action_cable_overview.md

@@ -582,6 +582,7 @@ run ActionCable.server
 ```
 
 Then you start the server using a binstub in bin/cable ala:
+
 ```
 #!/bin/bash
 bundle exec puma -p 28080 cable/config.ru