gpio: Fix a documentation spelling mistake

Simple etraneous letter.
Reported-by: NAlexander Stein <alexander.stein@systec-electronic.com>
Signed-off-by: NLinus Walleij <linus.walleij@linaro.org>

Documentation/gpio/board.txt

@@ -125,7 +125,7 @@ where
   - flags is defined to specify the following properties:
 	* GPIO_ACTIVE_HIGH	- GPIO line is active high
 	* GPIO_ACTIVE_LOW	- GPIO line is active low
-	* GPIO_OPEN_DRAIN	- GPIO line ise set up as open drain
+	* GPIO_OPEN_DRAIN	- GPIO line is set up as open drain
 	* GPIO_OPEN_SOURCE	- GPIO line is set up as open source
 	* GPIO_PERSISTENT	- GPIO line is persistent during
 				  suspend/resume and maintains its value