Devise guide - Add sign-up and login links: remove `protect_from_forgery` comment

[morinoko]

Devise guide tells students to

Open up app/controllers/application_controller.rb and add:
before_action :authenticate_user!
after protect_from_forgery with: :exception.

But protect_from_forgery with: :exception is not longer present in Rails 6, so this comment can be removed.

[vesan]

@morinoko You want to create a pull request of this change?

[morinoko]

@vesan Sorry it took so long, but I finally created a pull request: #439

[vesan]

Thanks! Merged the PR.