[1.7.x] Corrected example code for get_query_set upgrade in 1.6 release notes

The conditional setting of `get_query_set` is required for correct behaviour
if running Django 1.8. The full gory details are here:

http://lukeplant.me.uk/blog/posts/handling-django%27s-get_query_set-rename-is-hard/

Backport of f87e552d from master