fix admin show view

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | no
| Fixed tickets | #209
| License       | MIT
| Doc PR        | 

sonata doctrine phpcr admin did not properly handle relations in show mode. but additionally, this admin was trying to confuse the show mapper.

this change will only help together with
https://github.com/sonata-project/SonataDoctrinePhpcrAdminBundle/pull/307 - but we can merge, things won't get worse than before.
the content of the menu is a reference, not "text". this was the reason the problem was only detected in twig. when letting sonata figure out what field this is, it properly reports that we either need a __toString method or specify the associated_property.