use *decorate_collection* instead of *decorate* on "articles" array.

Latest version of *Draper* which is being installed in this tutorial (cause of gem 'draper' in Gemfile) does not have *decorate* for collections anymore! It has a special method for collections called *decorate_collection* which needs to be used!

- In this tutorial either things need to be updated based on the latest version of *Draper* or the previous version which was used originally in this tutorial should be referenced in Gemfile explicitly!

- As I'm going through this tutorial I try to update things based on latest *Draper* gem!

- Whichever way you prefer, of course :-)

Thanks,
Merge pull request #551 from Sam-Serpoosh/patch-1

use *decorate_collection* instead of *decorate* on "articles" array.