chore: Add SPDX header

Signed-off-by: Andy Scherzinger <info@andy-scherzinger.de>

lib/Reference/CreateCardReferenceProvider.php

@@ -1,5 +1,8 @@
 <?php
-
+/**
+ * SPDX-FileCopyrightText: 2023 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
 namespace OCA\Deck\Reference;
 
 use OCA\Deck\AppInfo\Application;