Documentation: fix sound related doc refs

Make sound doc refs valid.

Signed-off-by: Tom Saeger <tom.saeger@oracle.com>
Reviewed-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/sound/kernel-api/writing-an-alsa-driver.rst

@@ -2498,7 +2498,7 @@ Mic boost
 Mic-boost switch is set as “Mic Boost” or “Mic Boost (6dB)”.
 
 More precise information can be found in
-``Documentation/sound/alsa/ControlNames.txt``.
+``Documentation/sound/designs/control-names.rst``.
 
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