Fix typo in README
Fix typo in README

then/than

Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)

jlhawn/dockramp,jwhonce/dockramp,ankushagarwal/dockramp,jlhawn/dockramp,ankushagarwal/dockramp,jlhawn/dockramp,saromanov/dockramp,saromanov/dockramp,jimt/dockramp,beni55/dockramp,ankushagarwal/dockramp,jimt/dockramp,beni55/dockramp,jwhonce/dockramp