Add 2nd parameter (class) to getReference() call

Since Doctrine data fixture v2, getReference() requires a second parameter, being the class

docs/index.rst

@@ -169,7 +169,7 @@ exact same object via its name.
         {
             $userGroup = new Group('administrators');
             // this reference returns the User object created in UserFixtures
-            $userGroup->addUser($this->getReference(UserFixtures::ADMIN_USER_REFERENCE));
+            $userGroup->addUser($this->getReference(UserFixtures::ADMIN_USER_REFERENCE, User::class));
 
             $manager->persist($userGroup);
             $manager->flush();