config examples for ActionCable now use Rails.application.config.action_cable

Some existing examples used ActionCable.server.config but for
configuring allowed_request_origins that is overridden in development
mode. The correct place to set that is
Rails.application.config.action_cable which the ActionCable initializer
loads from. I thought the other two examples should be changed as well
just in case a default value that would override a configured value is
introduced for either log_tags or disable_request_forgery_protection in
the future.

actioncable/README.md

@@ -324,7 +324,7 @@ Rails.application.paths.add "config/cable", with: "somewhere/else/cable.yml"
 Action Cable will only accept requests from specified origins, which are passed to the server config as an array. The origins can be instances of strings or regular expressions, against which a check for match will be performed.
 
 ```ruby
-ActionCable.server.config.allowed_request_origins = ['http://rubyonrails.com', /http:\/\/ruby.*/]
+Rails.application.config.action_cable.allowed_request_origins = ['http://rubyonrails.com', /http:\/\/ruby.*/]
 ```
 
 When running in the development environment, this defaults to "http://localhost:3000".
@@ -332,7 +332,7 @@ When running in the development environment, this defaults to "http://localhost:
 To disable and allow requests from any origin:
 
 ```ruby
-ActionCable.server.config.disable_request_forgery_protection = true
+Rails.application.config.action_cable.disable_request_forgery_protection = true
 ```
 
 ### Consumer Configuration
@@ -374,7 +374,7 @@ App.cable = ActionCable.createConsumer()
 The other common option to configure is the log tags applied to the per-connection logger. Here's close to what we're using in Basecamp:
 
 ```ruby
-ActionCable.server.config.log_tags = [
+Rails.application.config.action_cable.log_tags = [
   -> request { request.env['bc.account_id'] || "no-account" },
   :action_cable,
   -> request { request.uuid }