Use elapsed() method on Instant in examples/async_tasks/async_compute…

….rs (#4497)

# Objective

Make the example a tiny bit more elegant

## Solution

Change one line

---

## Changelog

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- If applicable, organize changes under "Added", "Changed", or "Fixed" sub-headings
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## Migration Guide

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- If this PR is a breaking change (relative to the last release of Bevy), describe how a user might need to migrate their code to support these changes
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examples/async_tasks/async_compute.rs

@@ -56,7 +56,7 @@ fn spawn_tasks(mut commands: Commands, thread_pool: Res<AsyncComputeTaskPool>) {
                     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 Instant::now() - start_time < duration {
+                    while start_time.elapsed() < duration {
                         // Spinning for 'duration', simulating doing hard
                         // compute work generating translation coords!
                     }