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.
This seems somewhat unfortunate that this information is stated, but no solution is given. I do think that discussing catching a panic is somewhat nuanced and maybe too technical, but it leaves me yearning a bit here wondering how I can write safe C bindings.