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>