Add model attribute to the get_queryset example in docs

Tiny little change to documentation.

Currently, leaving both `model` and `queryset` attributes out when defining a ModelViewset causes an assertion failure:
```'base_name' argument not specified, and could not automatically determine the name from the viewset, as it does not have a '.model' or '.queryset' attribute.```

Perhaps this is an indication of an underlying issue (i.e. this assertion should be adjusted to account for a `get_queryset` method being present), but fixing the documentation as things currently stand might help avoid confusion.