Fix preview example in README
Fix preview example in README

Fixes https://github.com/Mange/roadie-rails/issues/19
rafaelfranca/roadie-rails,rafaelfranca/roadie-rails,tjschuck/roadie-rails,rafaelfranca/roadie-rails,Mange/roadie-rails,Mange/roadie-rails,Mange/roadie-rails,tjschuck/roadie-rails,Mange/roadie-rails,tjschuck/roadie-rails