Fix wrong example in README

It's necessary to make facts available to the example by using let, thus the change is actually making the example in the README work properly.

Closes: #51

README.md

@@ -65,6 +65,7 @@ describe 'myclass::debian' do
   }
 
   on_supported_os(test_on).each do |os, facts|
+    let (:facts) { facts }
     it { is_expected.to compile.with_all_deps }
   end
 end