Fix AssetChanged code documentation to mention the PostUpdate schedule (#19093)

# Objective

- Fix `AssetChanged` code documentation to mention the `PostUpdate`
schedule instead of the `Last` schedule

## Testing

- Trivial (code doc). Check `bevy_asset/src/lib.rs` in function
`init_asset` to see where this is scheduled:
```rust
           .add_systems(
                PostUpdate,
                Assets::<A>::asset_events
                    .run_if(Assets::<A>::asset_events_condition)
                    .in_set(AssetEvents),
            )
```

Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>

crates/bevy_asset/src/asset_changed.rs

@@ -106,7 +106,7 @@ impl<'w, A: AsAssetId> AssetChangeCheck<'w, A> {
 /// - Removed assets are not detected.
 ///
 /// The list of changed assets only gets updated in the [`AssetEventSystems`] system set,
-/// which runs in `Last`. Therefore, `AssetChanged` will only pick up asset changes in schedules
+/// which runs in `PostUpdate`. Therefore, `AssetChanged` will only pick up asset changes in schedules
 /// following [`AssetEventSystems`] or the next frame. Consider adding the system in the `Last` schedule
 /// after [`AssetEventSystems`] if you need to react without frame delay to asset changes.
 ///