Fix a broken link in README.md

The link to documentation about `rescue_from` used to point to this:

http://guides.rubyonrails.org/action_controller_overview.html#rescue_from

When it should be pointing to this:

http://guides.rubyonrails.org/action_controller_overview.html#rescue-from

(Note the hyphen instead of underscore at the end.)

Also, thanks for making such a great library! I look forward to using pundit instead of cancan.