Wrong capitalisation in annotation

This caused "php app/console doctrine:schema:update" to fail with this error:

```
[Doctrine\Common\Annotations\AnnotationException]                                                         
  [Semantical Error] The annotation "@Doctrine\ORM\Mapping\generatedValue" in property Belan\DemoBundle\En  
  tity\Group::$id does not exist, or could not be auto-loaded.
```