Require Rails::Engine instead of Rails::Railtie

This is the way turbo-rails is set up, and seems more correct.

Author: sfnelson

lib/katalyst/kpop/engine.rb

@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-require "rails/railtie"
+require "rails/engine"
 require "turbo-rails"
 
 module Katalyst