[data.shenzhen] Fix license identifier

src/ptbench/data/shenzhen/fold_7.py

 # SPDX-FileCopyrightText: Copyright © 2023 Idiap Research Institute <contact@idiap.ch>
 #
-# SPD.. GPL-3.0-or-later
+# SPDX-License-Identifier: GPL-3.0-or-later
 
 from .datamodule import DataModule