Fix markup directive in Action Cable templates

When converting Action Cable docs from RDoc to Markdown, these two
templates were accidentally included in the list of files converted (and
the `:markup:` directive added) because they incorrectly had an `.rb`
extension instead of `.rb.tt`.

This commit fixes the extension and removes the `:markup:` directive.

actioncable/lib/rails/generators/channel/templates/application_cable/channel.rb.tt

@@ -1,5 +1,3 @@
-# :markup: markdown
-
 module ApplicationCable
   class Channel < ActionCable::Channel::Base
   end

actioncable/lib/rails/generators/channel/templates/application_cable/connection.rb.tt

@@ -1,5 +1,3 @@
-# :markup: markdown
-
 module ApplicationCable
   class Connection < ActionCable::Connection::Base
   end