Use correct spelling of "U-Boot"

Correct spelling of "U-Boot" shall be used in all written text
(documentation, comments in source files etc.).

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Heiko Schocher <hs@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Minkyu Kang <mk7.kang@samsung.com>
diff --git a/doc/README.malta b/doc/README.malta
index c8db8a0..8614696 100644
--- a/doc/README.malta
+++ b/doc/README.malta
@@ -13,4 +13,4 @@
       reset
       flash-boot /path/to/u-boot/u-boot.bin
 
-  - You should now be able to reboot your Malta to a U-boot shell.
+  - You should now be able to reboot your Malta to a U-Boot shell.