Update defining_entities.rst

[Doctrine\Common\Annotations\AnnotationException]                                                                                                  
  [Type Error] Attribute "length" of @ORM\Column declared on property Tutorial\BlogBundle\Entity\Post::$title expects a(n) integer, but got string.
Merge pull request #206 from kasifi/patch-1

Update defining_entities.rst : expects a(n) integer, but got string