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.fec_mxc b/doc/README.fec_mxc
index 30e05da..ed7e47d 100644
--- a/doc/README.fec_mxc
+++ b/doc/README.fec_mxc
@@ -1,4 +1,4 @@
-U-boot config options used in fec_mxc.c
+U-Boot config options used in fec_mxc.c
 
 CONFIG_FEC_MXC
 	Selects fec_mxc.c to be compiled into u-boot. Can read out the