wandboard: README: Adjust the U-Boot proper binary name

After the conversion to DM the U-Boot proper binary name
is 'u-boot-dtb.img', so adjust it accordingly.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
diff --git a/board/wandboard/README b/board/wandboard/README
index e5170bc..f84f205 100644
--- a/board/wandboard/README
+++ b/board/wandboard/README
@@ -29,7 +29,7 @@
 
 - Flash the u-boot.img image into the SD card:
 
-sudo dd if=u-boot.img of=/dev/mmcblk0 bs=1k seek=69; sync
+sudo dd if=u-boot-dtb.img of=/dev/mmcblk0 bs=1k seek=69; sync
 
 - Insert the SD card into the slot located in the bottom of the board (same side
 as the mx6 processor)