rayon-demo: port nbody visualize to EventLoop::run_app

[cuviper]

Since #1191, we have this warning:

warning: use of deprecated method `winit::event_loop::EventLoop::<T>::run`: use `EventLoop::run_app` instead
   --> rayon-demo/src/nbody/visualize.rs:137:10
    |
137 |         .run(move |event, target| match event {
    |          ^^^
    |
    = note: `#[warn(deprecated)]` on by default

[mtantaoui]

Hello, just opened a pull request on this one: #1206