[1.11.x] Fixed #28233 -- Used a simpler example in the aggregation "cheat sheet" docs.

Backport of 49b9c89d40 from master

docs/topics/db/aggregation.txt

@@ -67,11 +67,11 @@ In a hurry? Here's how to do common aggregate queries, assuming the models above
     >>> Book.objects.all().aggregate(Max('price'))
     {'price__max': Decimal('81.20')}
 
-    # Cost per page
-    >>> from django.db.models import F, FloatField, Sum
-    >>> Book.objects.all().aggregate(
-    ...    price_per_page=Sum(F('price')/F('pages'), output_field=FloatField()))
-    {'price_per_page': 0.4470664529184653}
+    # Difference between the highest priced book and the average price of all books.
+    >>> from django.db.models import FloatField
+    >>> Book.objects.aggregate(
+    ...     price_diff=Max('price', output_field=FloatField()) - Avg('price')))
+    {'price_diff': 46.85}
 
     # All the following queries involve traversing the Book<->Publisher
     # foreign key relationship backwards.