Use get_user_model instead of User in Generating Tokens example

Because that's a better way of doing it.

docs/api-guide/authentication.md

@@ -162,12 +162,12 @@ The `curl` command line tool may be useful for testing token authenticated APIs.
 
 If you want every user to have an automatically generated Token, you can simply catch the User's `post_save` signal.
 
-    from django.contrib.auth.models import User
+    from django.contrib.auth import get_user_model
     from django.db.models.signals import post_save
     from django.dispatch import receiver
     from rest_framework.authtoken.models import Token
 
-    @receiver(post_save, sender=User)
+    @receiver(post_save, sender=get_user_model())
     def create_auth_token(sender, instance=None, created=False, **kwargs):
         if created:
             Token.objects.create(user=instance)