Fix require in readme example

Modern version of rails use `rails_helper` instead of `spec_helper` to pull in all rails dependencies. With the example as written, the code will not work.

README.md

@@ -30,7 +30,7 @@ Set up specs.
     $ vim spec/acceptance/orders_spec.rb
 
 ```ruby
-require 'spec_helper'
+require 'rails_helper'
 require 'rspec_api_documentation/dsl'
 
 resource "Orders" do