[Mailer][Mime] Update SMIME repository node description in configuration

Clarified the documentation for the S/MIME certificate repository configuration. It now specifies that the repository should be a service implementing `SmimeCertificateRepositoryInterface`.

Author: Spomky

DependencyInjection/Configuration.php

@@ -2350,7 +2350,7 @@ private function addMailerSection(ArrayNodeDefinition $rootNode, callable $enabl
                             ->info('S/MIME encrypter configuration')
                             ->children()
                                 ->scalarNode('repository')
-                                    ->info('Path to the S/MIME certificate repository. Shall implement the `Symfony\Component\Mailer\EventListener\SmimeCertificateRepositoryInterface`.')
+                                    ->info('S/MIME certificate repository service. This service shall implement the `Symfony\Component\Mailer\EventListener\SmimeCertificateRepositoryInterface`.')
                                     ->defaultValue('')
                                     ->cannotBeEmpty()
                                 ->end()