dist/tools/board_doc_check: change license header to SPDX format

Co-authored-by: crasbe <crasbe@gmail.com>

Author: zhqqzn

dist/tools/board_doc_check/check.sh

@@ -1,10 +1,7 @@
 #!/usr/bin/env bash
 #
-# Copyright (C) 2026 Evgeniy Kuznetsov <evvykuznetsov@edu.hse.ru>
-#
-# This file is subject to the terms and conditions of the GNU Lesser
-# General Public License v2.1. See the file LICENSE in the top level
-# directory for more details.
+# SPDX-FileCopyrightText: 2026 Evgeniy Kuznetsov <evvykuznetsov@edu.hse.ru>
+# SPDX-License-Identifier: LGPL-2.1-only
 
 : "${RIOTBASE:=$(cd "$(dirname "$0")"/../../../ || exit; pwd)}"
 : "${RIOTTOOLS:=${RIOTBASE}/dist/tools}"