Fix AssetChanged code documentation to mention the PostUpdate schedule

[Henauxg]

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:

           .add_systems(
                PostUpdate,
                Assets::<A>::asset_events
                    .run_if(Assets::<A>::asset_events_condition)
                    .in_set(AssetEvents),
            )