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

Documentation/sound/designs/procfile.rst

@@ -91,7 +91,7 @@ PCM Proc Files
 
 ``card*/pcm*/xrun_debug``
 	This file appears when ``CONFIG_SND_DEBUG=y`` and
-	``CONFIG_PCM_XRUN_DEBUG=y``.
+	``CONFIG_SND_PCM_XRUN_DEBUG=y``.
 	This shows the status of xrun (= buffer overrun/xrun) and
 	invalid PCM position debug/check of ALSA PCM middle layer.
 	It takes an integer value, can be changed by writing to this