Update README.md

For Rails 2.3.11 (and above?), I had better luck modifying the render block so that it prints the `path_without_format_and_extension` method instead of `@virtual_path`.

I figured out the method name by looking inside [this][rails]. I think `@virtual_path` is only used in Rails 3?

 [rails]: https://github.com/rails/rails/blob/v2.3.18/actionpack/lib/action_view/renderable_partial.rb#L19