This PR will fix the issue of doc is not automated rendered
through GitHub Actions anymore.
It looks the issue is cause by the fact that in `fixup_file_mtime.sh`
is only fixing files on source (.md) side, not on target (man/*.[1|5|7])
side. As a result Makefile will skip the rendering of doc as
it assume everything will be update to date.
This should fix the issue we were facing.
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>
