Update README.md

ActiveRecord -> Active Record [ci skip]

actioncable/README.md

@@ -5,7 +5,7 @@ It allows for real-time features to be written in Ruby in the same style
 and form as the rest of your Rails application, while still being performant
 and scalable. It's a full-stack offering that provides both a client-side
 JavaScript framework and a server-side Ruby framework. You have access to your full
-domain model written with ActiveRecord or your ORM of choice.
+domain model written with Active Record or your ORM of choice.
 
 
 ## Terminology