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)
Hehe, yes technology and grammar can conflict :smile: 

In Docker's case, I seriously doubt you'll have trouble getting top rankings on Google, so probably not a huge point. Perhaps something to add to the docs-guidelines, just for good measure.