Update add_url.sublime-snippet

This is the way I run my snippet for Flask.add_url_rule since Flask assumes the endpoint as the name of the view function. https://github.com/mitsuhiko/flask/blob/master/flask/app.py#L918

Adding the `${1:/path}` and `${2:route_name}` allows you to tab through the values that have to be set once the snippet is inserted.