Update auto-routing.rst

It appears that in RC1 of auto-routing-bundle the routes fail to generate unless the route basepath is set in the config.

Exception:
[Doctrine\ODM\PHPCR\Id\IdException]
Property "parent" mapped as ParentDocument may not be empty in document of class "Symfony\Cmf\Bundle\RoutingAutoBundle\Model\AutoRoute"

Simply adding the route basepath fixes this issue.

| Q             | A
| ------------- | ---
| Doc fix?      | Yes
| New docs? | No
| Applies to   | auto-routing-bundle RC1
| Fixed tickets | #555