Remove "s" from constraint as it's wrong path

Changes from 
    use Symfony\Component\Security\Core\Validator\Constraints as SecurityAssert;
to 
    use Symfony\Component\Security\Core\Validator\Constraint as SecurityAssert;

reference/constraints/UserPassword.rst

@@ -50,7 +50,7 @@ password:
        // src/Acme/UserBundle/Form/Model/ChangePassword.php
        namespace Acme\UserBundle\Form\Model;
        
-       use Symfony\Component\Security\Core\Validator\Constraints as SecurityAssert;
+       use Symfony\Component\Security\Core\Validator\Constraint as SecurityAssert;
 
        class ChangePassword
        {