Improve lint rake task example to return proper exit code (#1011)

This will make sure to return whatever the system call returned and it's useful to chain shell calls, etc.

GETTING_STARTED.md

@@ -994,6 +994,7 @@ namespace :factory_girl do
       end
     else
       system("bundle exec rake factory_girl:lint RAILS_ENV='test'")
+      exit $?.exitstatus
     end
   end
 end