runtime: Add threaded_scheduler to examples

[Darksonn]

It can be pretty confusing when the core_threads example does not call threaded_scheduler, since actually building such a runtime results in panics if you try to spawn something on it:

async fn a() {}

fn main() {
    let mut rt = runtime::Builder::new()
        .core_threads(4)
        .build()
        .unwrap();
        
    rt.spawn(a()); // panic
    rt.block_on(a());
}

playground

[carllerche]

Great work! thanks!