Fix urls() output

The docs were showing the files prefixed with the local directory path rather than the url prefix.

I tested to make sure this change is correct:

```python
>>> from webassets import Environment, Bundle
>>> env = Environment('static', '/foo/bar')
>>> bundle = Bundle('js/jquery.js', 'js/help.js', output='gen/packed.js')
>>> env.register('js', bundle)
<Bundle output=gen/packed.js, filters=(), contents=('js/jquery.js', 'js/help.js')>
>>> env['js'].urls()
['/foo/bar/gen/packed.js?ffa4c2ed']
>>> env.debug = True
>>> env['js'].urls()
['/foo/bar/js/jquery.js', '/foo/bar/js/help.js']
>>>
```