ASoC: wm8731: update wlf,wm8731.yaml reference

ASoC: wm8731: update wlf,wm8731.yaml reference

Changeset 8c21ffbb0018 ("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: 8c21ffbb0018 ("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.org
Signed-off-by: Mark Brown <broonie@kernel.org>