specs: Update spark spec docs to reference `quilt containers`
specs: Update spark spec docs to reference `quilt containers`

The spark spec docs were still suggesting `swarm ps`, but now,
we have an orchestrator-agnostic `quilt containers` command.

mmussomele/quilt,mmussomele/quilt,kklin/kelda,kayousterhout/quilt,NetSys/quilt,PenguinToast/quilt,mmussomele/quilt,stevenlin598/quilt,kayousterhout/quilt,ejj/quilt-dev,tejasmanohar/quilt,quilt/quilt,tejasmanohar/quilt,kklin/kelda,PenguinToast/quilt,quilt/quilt,LuiseV/quilt,LuiseV/quilt,kelda/kelda,kklin/kelda,kklin/quilt,kklin/quilt,kelda/kelda,kayousterhout/quilt,NetSys/quilt,stevenlin598/quilt,mmussomele/quilt,quilt/quilt,ejj/quilt-dev,mmussomele/quilt,LuiseV/quilt,PenguinToast/quilt,ejj/quilt-dev,kklin/kelda,kayousterhout/quilt,mmussomele/quilt,ejj/quilt-dev,kelda/kelda,PenguinToast/quilt,LuiseV/quilt,stevenlin598/quilt,kklin/quilt,LuiseV/quilt,tejasmanohar/quilt,tejasmanohar/quilt,NetSys/quilt,kelda/kelda,PenguinToast/quilt,stevenlin598/quilt,NetSys/quilt,stevenlin598/quilt,kklin/quilt,NetSys/quilt,stevenlin598/quilt,quilt/quilt