Update install.md
Update install.md

In zsh you can't use command line variables in url. By adding double quotation the install command will work with both zsh and bash.
londoncalling/docker.github.io,anweiss/docker.github.io,docker/docker.github.io,londoncalling/docker.github.io,thaJeztah/docker.github.io,londoncalling/docker.github.io,docker/docker.github.io,docker/docker.github.io,anweiss/docker.github.io,docker/docker.github.io,anweiss/docker.github.io,thaJeztah/docker.github.io,anweiss/docker.github.io,anweiss/docker.github.io,thaJeztah/docker.github.io,docker/docker.github.io,londoncalling/docker.github.io,thaJeztah/docker.github.io,londoncalling/docker.github.io,thaJeztah/docker.github.io