Add proper use of the password type

The documentation still referenced using the old `type`. 

Updated the code to use `PasswordType::class` vice `password`.

cookbook/doctrine/registration_form.rst

@@ -160,6 +160,7 @@ Next, create the form for the ``User`` entity::
     use Symfony\Component\Form\Extension\Core\Type\EmailType;
     use Symfony\Component\Form\Extension\Core\Type\TextType;
     use Symfony\Component\Form\Extension\Core\Type\RepeatedType;
+    use Symfony\Component\Form\Extension\Core\Type\PasswordType;
 
     class UserType extends AbstractType
     {
@@ -169,7 +170,7 @@ Next, create the form for the ``User`` entity::
                 ->add('email', EmailType::class)
                 ->add('username', TextType::class)
                 ->add('plainPassword', RepeatedType::class, array(
-                    'type' => 'password',
+                    'type' => PasswordType::class,
                     'first_options'  => array('label' => 'Password'),
                     'second_options' => array('label' => 'Repeat Password'),
                 )