Fix a typo in the instructions on using omap3's gpio interface.

Using the example for reading a gpio, shows the problem.
NULL should be the gpio number.

Signed-off-by: Tom Rix <Tom.Rix@windriver.com>
Acked-by: Dirk Behme <dirk.behme@googlemail.com>
diff --git a/doc/README.omap3 b/doc/README.omap3
index e05e816..66e781d 100644
--- a/doc/README.omap3
+++ b/doc/README.omap3
@@ -106,7 +106,7 @@
 To read a bit :
 
 	if (!omap_request_gpio(N)) {
-		omap_set_gpio_direction(NULL, 1);
+		omap_set_gpio_direction(N, 1);
 		val = omap_get_gpio_datain(N);
 		omap_free_gpio(N);
 	}