Fix minor bug in contrib/man/md/md2man-all.sh causing the creation of contrib/man/manREADME

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #6424 from tianon/fix-minor-md2man-bug

Fix minor bug in contrib/man/md/md2man-all.sh causing the creation of contrib/man/manREADME