fixes #447 - drop trailing slash from fetch docs

Sure enough, the [default tastypie base_urls](https://github.com/toastdriven/django-tastypie/blob/master/tastypie/resources.py#L311-320) allow the detail uri to contain `/` and `-` characters. So, `/api/tag/foo/` goes to `dispatch_detail` with resource_name: `tag` and slug: `foo/`

So, I think the best thing to do is to adopt a convention of not *officially* supporting trailing slashes for the detail nor set endpoints.