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/drivers/serial/sandbox.c b/drivers/serial/sandbox.c
index 45dff98..58f882b 100644
--- a/drivers/serial/sandbox.c
+++ b/drivers/serial/sandbox.c
@@ -25,7 +25,7 @@
 /*
  *
  *   serial_buf: A buffer that holds keyboard characters for the
- *		 Sandbox U-boot.
+ *		 Sandbox U-Boot.
  *
  * invariants:
  *   serial_buf_write		 == serial_buf_read -> empty buffer