Added Prefetch import to first use in QuerySet docs.

docs/ref/models/querysets.txt

@@ -1084,6 +1084,7 @@ the prefetch operation.
 In its simplest form ``Prefetch`` is equivalent to the traditional string based
 lookups:
 
+    >>> from django.db.models import Prefetch
     >>> Restaurant.objects.prefetch_related(Prefetch('pizzas__toppings'))
 
 You can provide a custom queryset with the optional ``queryset`` argument.
@@ -3138,6 +3139,7 @@ The ``lookup`` argument describes the relations to follow and works the same
 as the string based lookups passed to
 :meth:`~django.db.models.query.QuerySet.prefetch_related()`. For example:
 
+    >>> from django.db.models import Prefetch
     >>> Question.objects.prefetch_related(Prefetch('choice_set')).get().choice_set.all()
     <QuerySet [<Choice: Not much>, <Choice: The sky>, <Choice: Just hacking again>]>
     # This will only execute two queries regardless of the number of Question