Fix outdated reference to SimpleCmsBundle\Document\Page in document_tree config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.
Update class names in sonata_doctrine_phprcr_admin config

Classes were moved from Document to Doctrine\Phpcr namespace but tutorial config was not updated.
Merge pull request #256 from benglass/master

Fix reference to SimpleCmsBundle\Document\Page in document_tree config