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: NTom Rix <Tom.Rix@windriver.com>
Acked-by: NDirk Behme <dirk.behme@googlemail.com>

doc/README.omap3

@@ -106,7 +106,7 @@ To clear a bit :
 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);
 	}