Fix getMetadata return type

Signed-off-by: Louis Chemineau <louis@chmn.me>

lib/private/Metadata/FileMetadata.php

@@ -28,7 +28,7 @@ use OCP\DB\Types;
 /**
  * @method string getGroupName()
  * @method void setGroupName(string $groupName)
- * @method string getMetadata()
+ * @method array getMetadata()
  * @method void setMetadata(array $metadata)
  * @see \OC\Core\Migrations\Version240000Date20220404230027
  */