README needs updating

Usage description is no longer valid:

  source = "git::https://gitlab.developers.cam.ac.uk/uis/devops/infra/terraform/gcp-site-monitoring.git?ref=v3"

should be

  source  = "gitlab.developers.cam.ac.uk/uis/gcp-site-monitoring/devops"
  version = "~> 3.0"