Input: gpio-keys - fix a documentation index issue

gpio_keys.c now exists in the drivers/input/keyboard/ rather than
drivers/input/.
Signed-off-by: Song Qiang <songqiang.1304521@gmail.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

Documentation/devicetree/bindings/input/gpio-keys.txt

-Device-Tree bindings for input/gpio_keys.c keyboard driver
+Device-Tree bindings for input/keyboard/gpio_keys.c keyboard driver
 
 Required properties:
 	- compatible = "gpio-keys";