Tweaks to Dockerfile tutorial

Made a few tweaks to Dockerfile tutorial links and removed some cruft from the tutorial itself.

Docker-DCO-1.1-Signed-off-by: Fred Lifton <fred.lifton@docker.com> (github: fredlf)
Oh, that's a good, and embarrassingly obvious, idea. I'll change it.

Is there a problem with underscores in links I don't know about? I probably did that out of the habit of using underscores for spaces in file names in the olden days.