docs: marvell: Fix typo in file build.txt
docs: marvell: Fix typo in file build.txt

Replace "Uboot" with "Ubuntu".

Signed-off-by: Ding Tao <07dff04f1a3ec2456623b40cc3a82a04949e5843@qq.com>

achingupta/arm-trusted-firmware,achingupta/arm-trusted-firmware