Fix probs in sections 5.6 and 5.9; [ci skip]

[ci skip]
Section 5.6: Make it explicit for noobs that we're changing the create controller action, and adding new method to be used by it.

Section 5.9: Back in 5.2 we changed <%= form_for :article do |f| %>   to: <%= form_for :article, url: articles_path do |f| %>