Fix guide typos

Expanded to include two typos in debugging_rails_applications.txt . Included is a cumulative patch.

I've applied the patch for the debugging typos in the docrails project.

The benchmarking guide hasn't been finished or edited yet, so holding off on that patch for now.

We'd love more doc review - in the future, please check http://guides.rubyonrails.org/ to see which guides are finished, and post doc bugs/patches to the Rails Guides LH - http://rails.lighthouseapp.com/p.... Thanks!