README: fix `crt` and `key` files names
README: fix `crt` and `key` files names

They are named `gitlab_ci.{crt,key}`, not `gitlab.{crt,key}`.

adiferd/docker-gitlab-ci,youhong316/docker-gitlab-ci,adiferd/docker-gitlab-ci,sameersbn/docker-gitlab-ci,sameersbn/docker-gitlab-ci,youhong316/docker-gitlab-ci