fix phpdoc statement of getAlias()

src/Symfony/Component/DependencyInjection/ContainerBuilder.php

@@ -548,7 +548,7 @@ class ContainerBuilder extends Container implements TaggedContainerInterface
      *
      * @param string $id The service identifier
      *
-     * @return string The aliased service identifier
+     * @return Definition A Definition instance
      *
      * @throws \InvalidArgumentException if the alias does not exist
      *