ASoC: wm8731: update wlf,wm8731.yaml reference

Changeset 0e336eea ("ASoC: wm8731: Convert DT bindings to YAML format")
renamed: Documentation/devicetree/bindings/sound/wm8731.txt
to: Documentation/devicetree/bindings/sound/wlf,wm8731.yaml.

Update its cross-reference accordingly.

Fixes: 0e336eea ("ASoC: wm8731: Convert DT bindings to YAML format")
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/e56e54fe0ebb1b6e8dd2e245c398190016eb0a34.1654529011.git.mchehab@kernel.orgSigned-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/atmel-sam9x5-wm8731-audio.txt

@@ -16,7 +16,7 @@ Board connectors:
  * Line In Jack
 
 wm8731 pins:
-cf Documentation/devicetree/bindings/sound/wm8731.txt
+cf Documentation/devicetree/bindings/sound/wlf,wm8731.yaml
 
 Example:
 sound {