Using github.com/go-acme/lego/cmd/lego confuses go get. It fetches
github.com/go-acme/lego@v2.7.2+incompatible (among others) and
stumbles over a type error with that version.
Correcting the go get path to github.com/go-acme/lego/v3/cmd/lego
does the right thing.
fixes#993
