Update index.md
Update index.md

Bad link for the Docker EE UCP cli reference page.
Was:
[reference documentation](/reference/ucp/3.0/cli/install.md)
Changed to:
[reference documentation](/reference/ucp/3.1/cli/install.md)
londoncalling/docker.github.io,thaJeztah/docker.github.io,docker/docker.github.io,thaJeztah/docker.github.io,londoncalling/docker.github.io,thaJeztah/docker.github.io,docker/docker.github.io,docker/docker.github.io,docker/docker.github.io,londoncalling/docker.github.io,londoncalling/docker.github.io,thaJeztah/docker.github.io,thaJeztah/docker.github.io,londoncalling/docker.github.io,docker/docker.github.io