Fix timestamp issue for doc generating (#4683)

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>

.github/fixup_file_mtime.sh

@@ -9,7 +9,7 @@ if [[ ! -f 'coredns.1.md' ]]; then
   exit 1
 fi
 
-for file in coredns.1.md corefile.5.md plugin/*/README.md; do
+for file in coredns.1.md corefile.5.md plugin/*/README.md man/*.1 man/*.5 man/*.7; do
   time=$(git log --pretty=format:%cd -n 1 --date='format:%Y%m%d%H%M.%S' "${file}")
   touch -m -t "${time}" "${file}"
 done