Use std::thread::sleep instead of spin-waiting in the async_compute…

… example (#11856)

# Objective

- The [`async_compute`
example](https://github.com/bevyengine/bevy/blob/77c26f64ce0c1a538629a544b7020eb2d2b77466/examples/async_tasks/async_compute.rs#L65-L68)
uses a busy loop.
- It's good practice to mark busy loops with the
[`std::hint::spin_loop`](https://doc.rust-lang.org/stable/std/hint/fn.spin_loop.html)
function.

## Solution

- Call
[`std::hint::spin_loop`](https://doc.rust-lang.org/stable/std/hint/fn.spin_loop.html)
within the busy loop.

## Discussion

- While it is good practice to mark busy loops with `spin_loop`, it does
somewhat increase cognitive complexity. Since it is an example, it does
not matter too much.
- This is somewhat mitigated by the fact that it is within
[`std::hint`](https://doc.rust-lang.org/stable/std/hint/index.html),
which only affects compilation and doesn't do anything.
- Should I add a comment on what `spin_loop` does?
- Should the `while` loop just be replaced with `std::thread::sleep`?

examples/async_tasks/async_compute.rs

@@ -7,7 +7,7 @@ use bevy::{
     tasks::{block_on, futures_lite::future, AsyncComputeTaskPool, Task},
 };
 use rand::Rng;
-use std::time::{Duration, Instant};
+use std::{thread, time::Duration};
 
 fn main() {
     App::new()
@@ -60,12 +60,11 @@ fn spawn_tasks(mut commands: Commands) {
                 let entity = commands.spawn_empty().id();
                 let task = thread_pool.spawn(async move {
                     let mut rng = rand::thread_rng();
-                    let start_time = Instant::now();
+
                     let duration = Duration::from_secs_f32(rng.gen_range(0.05..0.2));
-                    while start_time.elapsed() < duration {
-                        // Spinning for 'duration', simulating doing hard
-                        // compute work generating translation coords!
-                    }
+
+                    // Pretend this is a time-intensive function. :)
+                    thread::sleep(duration);
 
                     // Such hard work, all done!
                     let transform = Transform::from_xyz(x as f32, y as f32, z as f32);