ASoC: audio-graph-card: update audio-graph-card.yaml reference

ASoC: audio-graph-card: update audio-graph-card.yaml reference

Changeset 97198614f6c3 ("ASoC: audio-graph-card: switch to yaml base Documentation")
renamed: Documentation/devicetree/bindings/sound/audio-graph-card.txt
to: Documentation/devicetree/bindings/sound/audio-graph-card.yaml.

Update its cross-reference accordingly.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/8a779e6b9644d19c5d77b382059f6ccf9781434d.1610535350.git.mchehab+huawei@kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>