Fix example default value of config.parent_controller in Devise initializer template

[penguin-10]

This PR fixes the commented example default value for config.parent_controller in
the Devise initializer template (config/initializers/devise.rb).

The previous example set config.parent_controller to DeviseController, which is
incorrect. The actual default parent controller is ApplicationController.

The default value is set here:

devise/lib/devise.rb

Lines 240 to 244 in cf93de3

	# The parent controller all Devise controllers inherits from.
	# Defaults to ApplicationController. This should be set early
	# in the initialization process and should be set to a string.
	mattr_accessor :parent_controller
	@@parent_controller = "ApplicationController"

This setting is used in the code here:
https://github.com/heartcombo/devise/blob/cf93de390a29654620fdf7ac07b4794eb95171d0/app/controllers/devise_controller.rb#L4C1-L4C62