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)
not a huge problem, but from a SEO perspective, dashes are preferred over underscores;
http://blog.woorank.com/2013/04/underscores-in-urls-why-are-they-not-recommended/
https://support.google.com/webmasters/answer/76329?hl=en

In *this* case, I think it's a bit inconsistent because both underscores *and* a dash is used