Update SonataOrderExtension.php

fixes that:
php app/console doctrine:schema:validate
[Mapping]  FAIL - The entity-class 'Application\Sonata\OrderBundle\Entity\Order' mapping is invalid:
* The field Application\Sonata\OrderBundle\Entity\Order#orderElements is on the inverse side of a bi-directional relationship, but the specified mappedBy association on the target-entity Application\Sonata\OrderBundle\Entity\OrderElement#order does not contain the required 'inversedBy=orderElements' attribute.