gpio.h: fix location of gpio legacy documentation

The location of this doc file was moved. Change its reference
accordingly.

Fixes: 7ee2c130 ("Documentation: gpio: Move legacy documentation to driver-api")
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

include/linux/gpio.h

@@ -14,7 +14,7 @@
 
 #include <linux/errno.h>
 
-/* see Documentation/gpio/gpio-legacy.txt */
+/* see Documentation/driver-api/gpio/legacy.rst */
 
 /* make these flag values available regardless of GPIO kconfig options */
 #define GPIOF_DIR_OUT	(0 << 0)