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.menu b/doc/README.menu
index ad520ab..cf14231 100644
--- a/doc/README.menu
+++ b/doc/README.menu
@@ -4,7 +4,7 @@
  * SPDX-License-Identifier:	GPL-2.0+
  */
 
-U-boot provides a set of interfaces for creating and using simple, text
+U-Boot provides a set of interfaces for creating and using simple, text
 based menus. Menus are displayed as lists of labeled entries on the
 console, and an entry can be selected by entering its label.