Which example to use to replace outdated reference in README to hello_world

[andrewdavidmackenzie]

I see the link in the readme to hello_world example is broken as hello_world doesn't exist any more...

I'm will put push a simple PR to fix that.

lazy_input looks a little more commented than calc, would that be the best one to use to replace hello_world?

Or add link to both (since there's only two)?