Use the correct option name in initialization: s/:force_runs/:force_run/

Referenced as :force_runs only in this one place. Referenced as :force_run in three places:
  ./lib/guard/rails/runner.rb:15
  ./README.md:37
  ./spec/lib/guard/rails/runner_spec.rb:210

No functionality changes as a result of this fix. Just a typo fix/clean up.