chore: Replace tokio sleep with background executor timer

[Vanuan]

Fix Async Patterns in GPUI Documentation

Description

This PR fixes the async spawn examples in skills/gpui/references/async.md that contained incorrect patterns leading to runtime panics. The examples were using tokio's sleep functions (tokio::time::sleep) within GPUI's async context, which is incorrect. The correct pattern is to use GPUI's built-in timer functionality via cx.background_executor().timer().

Problem Solved:

• Removed dependency on tokio for sleep functionality in GPUI async examples

Benefits:

• Example will run without panics
• Demonstrates the correct GPUI async pattern

Screenshot

Before	After
[Runtime panic on async spawn]	[Successfully runs and updates UI]

Break Changes

None. This is purely a documentation fix that corrects the example code.

---

Correct Async Pattern Example:

- // ❌ Incorrect: Uses tokio sleep and wrong spawn signature
- cx.spawn(...{
-     tokio::time::sleep(std::time::Duration::from_secs(2)).await;
-     cx.update(|cx| {
...
-     }).ok();
- });

+ // ✅ Correct: Uses GPUI's timer and proper spawn signature
+ cx.spawn(async move |this, cx| {
+     cx.background_executor()
+         .timer(std::time::Duration::from_secs(2))
+         .await;
+     
+     this.update(cx, |this, cx| {
...
+     }).ok();
+ });

[huacnlee]

Thanks.