[3.0.x] Clarified AppConfig.ready() docs example.

Backport of 971a84d6af from master

docs/ref/applications.txt

@@ -256,8 +256,13 @@ Methods
 
     Example::
 
+        from django.apps import AppConfig
         from django.db.models.signals import pre_save
 
+
+        class RockNRollConfig(AppConfig):
+            # ...
+
             def ready(self):
                 # importing model classes
                 from .models import MyModel  # or...