Update the_model.rst

When installing via composer as per the documentation, the example does not work with phpcr-odm 1.0. It is necessary to add use Doctrine\Common\DataFixtures\OrderedFixtureInterface;, implement both the FixtureInterface and OrderedFixtureInterface classes and specify a getOrder function with a value larger than that in LoadSimpleCmsData.php, otherwise the following error message is thrown when running php app/console running doctrine:phpcr:fixtures:load:

Property "parent" mapped as ParentDocument may not be empty in document of
class "Symfony\Cmf\Bundle\SimpleCmsBundle\Doctrine\Phpcr\Page"