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: https://github.com/heartcombo/devise/blob/cf93de390a29654620fdf7ac07b4794eb95171d0/lib/devise.rb#L240-L244

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