ALSA: doc: Fix reference to mixart.rst

MIXART.txt has been converted to ReST and renamed. Fix the reference
in alsa-configuration.rst.

Fixes: 3d8e8186 ("ALSA: doc: ReSTize MIXART.txt")
Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20210101221942.1068388-1-j.neuschaefer@gmx.net


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