Fix reference to SimpleCmsBundle\Document\Page in document_tree config

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | e77a72b0c7999509be31852e838e156632ca85fd and above
| Fixed tickets | https://github.com/sonata-project/SonataDoctrinePhpcrAdminBundle/issues/168

The config for the sonata_admin_phpcr_admin.document_tree contained a mapping for a class which has been removed (SimpleCmsBundle\Document\Page). This class has been replaced with 
Symfony\Cmf\Bundle\SimpleCmsBundle\Doctrine\Phpcr\Page and this mapping is required in order for Sonata Admin to display and allow editing of the default documents created by the SimpleCmsBundle in the PHPCR.