Add header demarcation to Action Cable guide

Guide generation is configured to detect a guide header by a 40-or-more-hyphen long line. The Action Cable guide was missing this line, so the page title on the guides site was only showing "Ruby on Rails Guides", and the section title in the Kindle index was "Ruby on Rails Guides". Adding the header demarcation line back in fixes both of these.

Fixes #25697

[ci skip]

guides/source/action_cable_overview.md

@@ -9,7 +9,9 @@ After reading this guide, you will know:
 * What Action Cable is and its integration on backend and frontend  
 * How to setup Action Cable
 * How to setup channels
-* Deployment and Architecture setup for running Action Cable 
+* Deployment and Architecture setup for running Action Cable
+
+--------------------------------------------------------------------------------
 
 Introduction
 ------------