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/cmd/spibootldr.c b/cmd/spibootldr.c
index ca76dde..acbb0f6 100644
--- a/cmd/spibootldr.c
+++ b/cmd/spibootldr.c
@@ -1,5 +1,5 @@
 /*
- * U-boot - spibootldr.c
+ * U-Boot - spibootldr.c
  *
  * Copyright (c) 2005-2008 Analog Devices Inc.
  *