gpio: update path to documentation

Signed-off-by: Richard Genoud <richard.genoud@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

include/linux/gpio.h

@@ -3,7 +3,7 @@
 
 #include <linux/errno.h>
 
-/* see Documentation/gpio.txt */
+/* see Documentation/gpio/gpio-legacy.txt */
 
 /* make these flag values available regardless of GPIO kconfig options */
 #define GPIOF_DIR_OUT	(0 << 0)