[1.7.x] Fixed #24789 -- Fixed wrong positional args order in doc example

Arguments shown in example code (signal, sender, instance) appeared to
be the incorrect positional arguments for a post_save signal (which
might start as: sender, instance, created), as documented:
​https://docs.djangoproject.com/en/1.8/ref/signals/#post-save

Backport of 4f3c44424187de20d7f75fdde6624b9f683a9cf2 from master.

docs/topics/auth/customizing.txt

@@ -446,7 +446,7 @@ different User model.
             from django.conf import settings
             from django.db.models.signals import post_save
 
-            def post_save_receiver(signal, sender, instance, **kwargs):
+            def post_save_receiver(sender, instance, created, **kwargs):
                 pass
 
             post_save.connect(post_save_receiver, sender=settings.AUTH_USER_MODEL)