ASoC: simple-card: Fix detect gpio documentation.

The device tree property uses '-' not '_'.
Signed-off-by: Dylan Reid <dgreid@chromium.org>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/simple-card.txt

@@ -17,9 +17,9 @@ Optional properties:
 					  source.
 - simple-audio-card,mclk-fs             : Multiplication factor between stream rate and codec
   					  mclk.
-- simple-audio-card,hp_det_gpio		: Reference to GPIO that signals when
+- simple-audio-card,hp-det-gpio		: Reference to GPIO that signals when
 					  headphones are attached.
-- simple-audio-card,mic_det_gpio	: Reference to GPIO that signals when
+- simple-audio-card,mic-det-gpio	: Reference to GPIO that signals when
 					  a microphone is attached.
 
 Optional subnodes: