Docs/sound/index: Add missing SPDX License Identifier

Add missing SPDX License Identifier for sound documentation index file.
Signed-off-by: SeongJae Park <sj@kernel.org>
Acked-by: Takashi Iwai <tiwai@suse.de>
Link: https://lore.kernel.org/r/20230122213650.187710-9-sj@kernel.orgSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/sound/index.rst

+.. SPDX-License-Identifier: GPL-2.0
+
 =============================
 Sound Subsystem Documentation
 =============================