binman: fix a few typos in documentation

This fixes four small typos in the README file.

Signed-off-by: Michael Heimpold <mhei@heimpold.de>
Cc: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>