Fix custom Roles in entity_provider cookbook.

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | Symfony version 2.0
| Fixed tickets | I don't believe this fixes any PR's, but does relate to many rejected PR's on symfony/symfony (listed below)


The current documentation seems to assume the implementation present in commit
https://github.com/symfony/symfony/pull/1673, which was reverted soon after its addition due to a potential, but undisclosed security hole (citation @schmittjoh in https://github.com/symfony/symfony/commit/af70ac8d777873c49347ac828a817a400006cbea).

This incorrect documentation has likely been the source of many
of the following issues:
* symfony/symfony#1538 - [ACL RoleSecurityIdentity] check if instance of Role
* symfony/symfony#1748 - Replace Role to RoleInterface for RoleSecurityIdentity
* symfony/symfony#4309 - Issue related to custom group (role) and ACL/ACE
* symfony/symfony#5026 - potential bug in Symfony\Component\Security\Acl\Domain\RoleSecurityIdentity
* symfony/symfony#5076 - [Acl] altered the behaviour of RoleSecurityIdentity
* symfony/symfony#5171 - Fix/role security identity
* symfony/symfony#5303 - [Security] Check for RoleInterface instead of Role object in RoleSecurityIdentity
* symfony/symfony#5909 - Allow Custom Roles to implement the RoleInterface
* symfony/symfony#6012 - Securityidentity fix