Add a use of route_service with ServeFile to the static-file-server example

[uckelman]

Motivation

The docs for Router::route_service give an example of using ServeFile, but if you come to the static file server example first, you won't necessarily be aware of route_service.

Solution

This PR adds an example using route_service, to make it easier to spot for people interested in serving static files.

[davidpdrsn]

Looks good but you have to run cargo fmt

[uckelman]

Huh, I'm surprised it wants that all on one line.