Update staticfile.rs Example

I'm interested in using `tide`, but I kinda really need to be able to serve static files. The [archived example](https://github.com/http-rs/tide/blob/4aec5fe2bb6b8202f7ae48e416eeb37345cf029f/backup/examples/staticfile.rs) was great, but it's now outdated and doesn't compile. Could we work on updating this?

Thanks!