Update docs to note that it installs as `eztables`, not `django-eztables`

I updated the docs to note that it installs as `eztables`, not `django-eztables`.

If you add `django-eztables` to your `INSTALLED_APPS`, you'll get an error:

```
Error: No module named django-eztables
```

Which makes sense because `-` isn't valid in a python module name.