ALSA: doc: use correct config variable name

CONFIG_PCM_XRUN_DEBUG should be CONFIG_SND_PCM_XRUN_DEBUG

Signed-off-by: Julia Lawall <Julia.Lawall@inria.fr>

Link: https://lore.kernel.org/r/1596223701-7558-1-git-send-email-Julia.Lawall@inria.fr


Signed-off-by: Takashi Iwai <tiwai@suse.de>