message: Fix link in docs to Ubuntu Dockerfile
diff: @@ -24,5 +24,5 @@ Docker
  ******
  Docker images including building and installation of ADIOS 2 in a container:
  -* Ubuntu 18.04: under `scripts/docker/images/ubuntu18.04/Dockerfile <https://github.com/ornladios/ADIOS2/tree/master/scripts/docker/images/ubuntu18.04/Dockerfile>`_
  +* Ubuntu 18.04: under `scripts/docker/images/ubuntu/Dockerfile <https://github.com/ornladios/ADIOS2/tree/master/scripts/docker/images/ubuntu/Dockerfile>`_
  