Correct inaccuracy in docker pull documentation

debian:testing most definitely does not point at ubuntu:14.04.
Perhaps the removed lines should be slightly rewritten and put beneath the `debian` example (replace `debian` with `ubuntu`) ie;

    $ sudo docker pull ubuntu
    # Will pull the `ubuntu:latest` image, and `ubuntu:trusty` and `ubuntu:14.04`
    # which are aliases of the same image
    # and any intermediate layers it is based on.

Although pulling aliases won't work with Registry V2 so not sure how much value
there is in keeping this in.