Update chapters/05-exercise-2.md

In the latest v0.9.10, Backbone changed the way to send attributes (not with the options parameter) to a third-party API. 

Now it's better with the attrs parameter.

This example is based in our implementation with Joomla CMS.

Thanks,
Merge pull request #291 from anibalsanchez/patch-1

Update chapters/05-exercise-2.md - Third-party Url Customization in the REST API