Update installation.rst

We must include "use Doctrine\ORM\EntityManager" in ItemResolver otherwise it will try to resolve EntityManager manager in the current namespace of App/AppBundle/Cart/. 

Without 'use Doctrine\ORM\EntityManager' the following error is produced: PHP Catchable fatal error:  Argument 1 passed to app\AppBundle\Cart\ItemResolver::__construct() must be an instance of app\AppBundle\Cart\EntityManager, instance of EntityManager51cc9f23a10c7_546a8d27f194334ee012bfe64f629947b07e4919\\__CG__\\Doctrine\\ORM\\EntityManager given.