add missing import in tests

Author: Paul Hallett

docs/api-guide/testing.md

@@ -65,6 +65,8 @@ When testing views directly using a request factory, it's often convenient to be
 
 To forcibly authenticate a request, use the `force_authenticate()` method.
 
+    from rest_framework.tests import force_authenticate
+
     factory = APIRequestFactory()
     user = User.objects.get(username='olivia')
     view = AccountDetail.as_view()