sandbox: Fix README to indicate that vendor name is unset

This brings in a additional small fix which was missed in a recent update
to the README.

Suggested-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Jagannadha Sutradharudu Teki <jagannadh.teki@gmail.com>