Remove broken link to md2roff

[hanstdam]

md2roff no longer exist in the referenced location.

$ curl -sI https://github.com/nereusx/md2roff | grep -i '^location:'
location: https://github.com/testtt12321327121/md2roff

This PR is to remove the reference while still providing useful guidance to contributors.