Fix README spec

Mocked types cannot be private.
Moved to a module to prevent polluting the global namespace.

spec/docs/readme_spec.cr

@@ -1,26 +1,28 @@
 require "../spec_helper"
 
-private abstract class Interface
+module Readme
+  abstract class Interface
     abstract def invoke(thing) : String
-end
+  end
 
-# Type being tested.
+  # Type being tested.
-private class Driver
+  class Driver
     def do_something(interface : Interface, thing)
       interface.invoke(thing)
     end
+  end
 end
 
-Spectator.describe Driver do
+Spectator.describe Readme::Driver do
   # Define a mock for Interface.
-  mock Interface
+  mock Readme::Interface
 
   # Define a double that the interface will use.
   double(:my_double, foo: 42)
 
   it "does a thing" do
     # Create an instance of the mock interface.
-    interface = mock(Interface)
+    interface = mock(Readme::Interface)
     # Indicate that `#invoke` should return "test" when called.
     allow(interface).to receive(:invoke).and_return("test")