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/arch/x86/cpu/resetvec.S b/arch/x86/cpu/resetvec.S
index 68a5b94..183275b 100644
--- a/arch/x86/cpu/resetvec.S
+++ b/arch/x86/cpu/resetvec.S
@@ -1,5 +1,5 @@
 /*
- *  U-boot - x86 Startup Code
+ *  U-Boot - x86 Startup Code
  *
  * (C) Copyright 2002
  * Daniel Engström, Omicron Ceti AB, <daniel@omicron.se>