fix caps of @ORM\PreUpdate

[glensc]

current symfony2/doctrine is picky about the case:

$ php app/console doctrine:generate:entities 'Blogger\BlogBundle'
Generating entities for namespace "Blogger\BlogBundle"

  [Doctrine\Common\Annotations\AnnotationException]                                                                                                           
  [Semantical Error] The annotation "@Doctrine\ORM\Mapping\preUpdate" in method Blogger\BlogBundle\Entity\Comment::setUpdatedValue() does not exist, or could not be auto-loaded.