Fix contrib/man/md/docker-import.1.md warning ("macro `tar.gz,' not defined")

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #6426 from tianon/fix-import-man-error

Fix contrib/man/md/docker-import.1.md warning ("macro `tar.gz,' not defined")