update to be compatible with current sonata admin


| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

we need a toString method on documents and the symfony forms explode if publishable is not a boolean.