docs: marvell: Fix typo in file build.txt

Replace "Uboot" with "Ubuntu".

Signed-off-by: Ding Tao <miyatsu@qq.com>
diff --git a/docs/marvell/build.txt b/docs/marvell/build.txt
index 0682b77..a00dbb5 100644
--- a/docs/marvell/build.txt
+++ b/docs/marvell/build.txt
@@ -167,7 +167,7 @@
 
 (1) ARM cross compiler capable of building images for the service CPU (CM3).
     This component is usually included in the Linux host packages.
-    On Debian/Uboot hosts the default GNU ARM tool chain can be installed
+    On Debian/Ubuntu hosts the default GNU ARM tool chain can be installed
     using the following command::
 
 		> sudo apt-get install gcc-arm-linux-gnueabi