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/board/br4/br4.c b/board/br4/br4.c
index bc034e3..6f3f170 100644
--- a/board/br4/br4.c
+++ b/board/br4/br4.c
@@ -1,5 +1,5 @@
 /*
- * U-boot - main board file
+ * U-Boot - main board file
  *
  * Copyright (c) Switchfin Org. <dpn@switchfin.org>
  *