Fix undefined ApplicationDeliveryMethod (#470)

* Fix undefined ApplicationDeliveryMethod

* Update README.md

Author: adrienpoly

README.md

@@ -525,7 +525,7 @@ If you want to build your own delivery method to deliver notifications to a spec
 
 `rails generate noticed:delivery_method Discord`
 
-This will generate a new `DeliveryMethods::Discord` class inside the `app/notifiers/delivery_methods` folder, which can be used to deliver notifications to Discord.
+This will generate a new `ApplicationDeliveryMethod` and `DeliveryMethods::Discord` class inside the `app/notifiers/delivery_methods` folder, which can be used to deliver notifications to Discord.
 
 ```ruby
 class DeliveryMethods::Discord < ApplicationDeliveryMethod