Added "How to create an event subscriber"

I've added the information about how to create an event subscriber to the "How to create an Event listener" cookbook recipe. I think that there isn't a recipe in the cookbook about that and it can be useful.
Here you use "Event Listener" and "Event Subscriber", but in the rest of the article it's "event listener" and "event subscriber". Is there any reason for the capitalization of letters in this case?