nitrogen6x: tweak readme wording

The filesystem is directly written to the card, no extraction is performed.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

board/boundarydevices/nitrogen6x/readme.txt

@@ -2,7 +2,7 @@ Buildroot for Boundary Devices Nitrogen6X:
 
 http://boundarydevices.com/products/nitrogen6x-board-imx6-arm-cortex-a9-sbc/
 
-To install, simply extract rootfs.ext2 to first partition of a uSD card:
+To install, simply write rootfs.ext2 to the first partition of a uSD card:
 
 sudo dd if=output/images/rootfs.ext2 of=/dev/sdX1