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: NJonathan Neuschäfer <j.neuschaefer@gmx.net>
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20210101221942.1068388-1-j.neuschaefer@gmx.netSigned-off-by: NTakashi Iwai <tiwai@suse.de>

Documentation/sound/alsa-configuration.rst

@@ -1501,7 +1501,7 @@ Module for Digigram miXart8 sound cards.
 
 This module supports multiple cards.
 Note: One miXart8 board will be represented as 4 alsa cards.
-See MIXART.txt for details.
+See Documentation/sound/cards/mixart.rst for details.
 
 When the driver is compiled as a module and the hotplug firmware
 is supported, the firmware data is loaded via hotplug automatically.