Fix cookie serializer docs

The option actually needs to be set on action_dispatch. [ci skip]

Conflicts:
	guides/source/upgrading_ruby_on_rails.md

guides/source/upgrading_ruby_on_rails.md

@@ -104,9 +104,9 @@ Applications created before Rails 4.1 uses `Marshal` to serialize cookie values
 the signed and encrypted cookie jars. If you want to use the new `JSON`-based format
 in your application, you can add an initializer file with the following content:
 
-  ```ruby
-  Rails.application.config.cookies_serializer :hybrid
-  ```
+```ruby
+Rails.application.config.action_dispatch.cookies_serializer = :hybrid
+```
 
 This would transparently migrate your existing `Marshal`-serialized cookies into the
 new `JSON`-based format.