bug #6324 Fixed UserCheckerInterface importing (VIs-a-vis)

This PR was submitted for the 3.0 branch but it was merged into the 2.8 branch instead (closes #6324).

Discussion
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Fixed UserCheckerInterface importing

Fixed UserCheckerInterface name in ``use`` statement

Commits
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25b3aed Update user_checkers.rst

Author: wouterj

Diff for: cookbook/security/user_checkers.rst

@@ -29,7 +29,7 @@ are not met, an exception should be thrown which extends the
     use AppBundle\Exception\AccountDeletedException;
     use AppBundle\Security\User as AppUser;
     use Symfony\Component\Security\Core\Exception\AccountExpiredException;
-    use Symfony\Component\Security\Core\User\UserCheckInterface;
+    use Symfony\Component\Security\Core\User\UserCheckerInterface;
     use Symfony\Component\Security\Core\User\UserInterface;
 
     class UserChecker implements UserCheckerInterface