Corrected FOSUserEvents.php comment pointing to wrong event

 /**
     * The REGISTRATION_INITIALIZE event occurs when the registration process is initialized.
     *
     * This event allows you to modify the default values of the user before binding the form.
     * The event listener method receives a FOS\UserBundle\Event\GetResponseUserEvent instance.
     */
    const REGISTRATION_INITIALIZE = 'fos_user.registration.initialize';


REGISTRATION_INITIALIZE event listeners actually receive a UserEvent object as detailed in this line of code from RegistrationController.php line 46:

$dispatcher->dispatch(FOSUserEvents::REGISTRATION_INITIALIZE, new UserEvent($user, $request));