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.
Technically you can link to a mangled function, it's just hard to guess the name! You may want to rephrase to emphasize that `#[no_mangle]` is purely just making the symbol known to the programmer (preventing mangling).