[1.7.x] Fixed #24789 -- Fixed wrong positional args order in doc example

Arguments shown in example code (signal, sender, instance) appeared to
be the incorrect positional arguments for a post_save signal (which
might start as: sender, instance, created), as documented:
​https://docs.djangoproject.com/en/1.8/ref/signals/#post-save

Backport of 4f3c4442 from master.