Fix example path params

Resolves the following error:

    thread 'main' (6023907) panicked at examples/live_federation/main.rs:58:10:
    Path segments must not start with `:`. For capture groups, use `{capture}`. If you meant to literally match a segment starting with a colon, call `without_v07_checks` on the router.

examples/live_federation/main.rs

@@ -55,8 +55,8 @@ async fn main() -> Result<(), Error> {
     info!("Listen with HTTP server on {BIND_ADDRESS}");
     let config = config.clone();
     let app = Router::new()
-        .route("/:user", get(http_get_user))
-        .route("/:user/inbox", post(http_post_user_inbox))
+        .route("/{user}", get(http_get_user))
+        .route("/{user}/inbox", post(http_post_user_inbox))
         .route("/.well-known/webfinger", get(webfinger))
         .layer(FederationMiddleware::new(config));