minor #5938 Add proper use of the password type (themccallister)

This PR was submitted for the 3.0 branch but it was merged into the 2.8 branch instead (closes #5938).

Discussion
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Add proper use of the password type

The documentation still referenced using the old `type`.

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

Commits
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7bd1f22 Add proper use of the password type

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'),
                 )