Fix Phpcr model

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | na
| Fixed tickets | https://github.com/symfony-cmf/MediaBundle/issues/13
| License       | MIT
| Doc PR        | na

While updating I found the model didn't work so this is directly fixed.