board/atmel/xplained/readme.txt: fix u-boot filename

The file that is actually produced is called u-boot-spl.bin, not
uboot-spl.bin.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

board/atmel/xplained/readme.txt

@@ -52,7 +52,7 @@ sudo mount /dev/mmcblk0p2 /media/rootfs
 Copy the bootloaders, kernel image and Device Tree blob to the first
 partition:
 
-cp output/images/uboot-spl.bin /media/boot/boot.bin
+cp output/images/u-boot-spl.bin /media/boot/boot.bin
 cp output/images/u-boot.img /media/boot/u-boot.img
 cp output/images/zImage /media/boot/zImage
 cp output/images/at91-sama5d3_xplained.dtb /media/boot/at91-sama5d3_xplained.dtb