Add an example to the FFI guide

This is a little short, but the FFI guide needs a cohesive go-over, and this at least adds the information in the meantime.

It also modernizes the style in a small way, by moving the tilde syntax to the grave syntax.
Agreed. I see this patch as a small improvement, but there are bigger things that can be done, if I had the time. Going into this would be one of those things.