* Update get_object() example in permissions.md 

I'm a bit confused about the example that's provided in the 'Object level permissions' section.  Other examples (e.g. Tutorial 3 - Class Based Views) provided a pk to get_object().  It doesn't seem like this example has any way of identifying a specific object.  

Just in case I'm correct, I've prepared this pull request. But if I'm wrong, would it be possible for you to explain the example I modified?  

Many Thanks...

* Adjust patch