Fix custom Roles in entity_provider cookbook.

The documentation seems to assume the implementation present in commit
https://github.com/symfony/symfony/pull/1673, which reverted soon after 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
Merge pull request #2541 from m14t/roleVsRoleInterfaceBug

Fix custom Roles in entity_provider cookbook.