README: Fix typo 'git-publish'

Reported-by: Alberto Garcia <berto@igalia.com>
Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20180306024328.19195-1-famz@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>