fixes #described_recipe helper

This was broken when using nested example groups because it would fail to traverse up to the top-level example, which is what I feel the correct behavior should be.

```ruby
describe "cookbook::recipe" do
  describe "some aspect" do
    it 'does something' do
      # #described_recipe helper returned 'does something'
    end
  end
end
```

I wrote a test to expose the bug and implemented a fix.