Fix cli argument usage typo for docker --mtu

Docker-DCO-1.1-Signed-off-by: Danny Berger <dpb587@gmail.com> (github: dpb587)
Drop -mtu from docs in favor of --mtu only

Docker-DCO-1.1-Signed-off-by: Danny Berger <dpb587@gmail.com> (github: dpb587)
Merge pull request #4280 from dpb587/bugfix-mtu-typo

Fix argument usage typo for docker --mtu