docs: .create => .create_user (#623)

Otherwise the test does not work.

Author: jakobdo

docs/helpers.rst

@@ -146,7 +146,7 @@ To use `client` as an authenticated standard user, call its `login()` method bef
     def test_with_authenticated_client(client, django_user_model):
         username = "user1"
         password = "bar"
-        django_user_model.objects.create(username=username, password=password)
+        django_user_model.objects.create_user(username=username, password=password)
         client.login(username=username, password=password)
         response = client.get('/private')
         assert response.content == 'Protected Area'