README: Fix Scope definition example

On Ruby 2.0.0 and Rails 4.0.0:

    Foo = Struct.new(:foo) do
      Bar = Struct.new(:bar) do
      end
    end

irb: `warning: toplevel constant Bar referenced by Foo::Bar`
Rails doesn't find the contant

We need to either use a class definition for `Foo` (`class Foo < Struct.new(:foo)`), or explicitly encapslulate `Bar`:

    Foo = Struct.new(:foo) do
      self::Bar = Struct.new(:bar) do
      end
    end