Input: sun4i-lradc-keys - fix typo in binding documentation

Trivial typo fix ("mut" -> "must") in the sunxi LRADC-keys binding
documentation.

Signed-off-by: Karsten Merker <merker@debian.org>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

Documentation/devicetree/bindings/input/sun4i-lradc-keys.txt

@@ -12,7 +12,7 @@ Each key is represented as a sub-node of "allwinner,sun4i-a10-lradc-keys":
 Required subnode-properties:
 	- label: Descriptive name of the key.
 	- linux,code: Keycode to emit.
-	- channel: Channel this key is attached to, mut be 0 or 1.
+	- channel: Channel this key is attached to, must be 0 or 1.
 	- voltage: Voltage in µV at lradc input when this key is pressed.
 
 Example: