mmc: sdhci-st: correct name of sd-uhs-sdr50 property

Correct what appears to be a typo in the name of the sd-uhs-sdr50.

Also fix mixed tab/space indentation.
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>

Documentation/devicetree/bindings/mmc/sdhci-st.txt

@@ -48,7 +48,7 @@ Optional properties:
 - vqmmc-supply:		Phandle to the regulator dt node, mentioned as the vcc/vdd
 			supply in eMMC/SD specs.
 
-- sd-uhs--sdr50:	To enable the SDR50 in the mmcss.
+- sd-uhs-sdr50:	To enable the SDR50 in the mmcss.
 			See:  Documentation/devicetree/bindings/mmc/mmc.txt.
 
 - sd-uhs-sdr104:	To enable the SDR104 in the mmcss.