Fix example code in README

- A method that has `?` suffix should return boolean, but this method return an object that respond to `valid?`.
- All other examples don't have `?`.

README.md

@@ -12,7 +12,7 @@ require "scientist"
 class MyWidget
  def allows?(user)
  experiment = Scientist::Default.new "widget-permissions"
- experiment.use { model.check_user?(user).valid? } # old way
+ experiment.use { model.check_user(user).valid? } # old way
  experiment.try { user.can?(:read, model) } # new way
 
  experiment.run