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.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>