[#5993] fix doubled "interface"

Author: xabbuh

cookbook/security/entity_provider.rst

@@ -460,7 +460,7 @@ interface only requires one method: ``loadUserByUsername($username)``::
 
 .. versionadded:: 2.8
     The :class:`Symfony\\Bridge\\Doctrine\\Security\\User\\UserLoaderInterface`
-    interface was introduced in 2.8. Prior to Symfony 2.8, you had to implement
+    was introduced in 2.8. Prior to Symfony 2.8, you had to implement
     ``Symfony\Component\Security\Core\User\UserProviderInterface``.
 
 .. tip::