Update log_layers_ecs example for children macro (#18293)

# Objective

Contributes to #18238 
Updates the `log_layers_ecs`, example to use the `children!` macro.

Note that I did not use a macro, nor `Children::spawn` for the outer
layer. Since the `EventReader` is borrowed mutably, any `.map` I did on
`events.read()` was going to have the reference outlive the function
body. I believe this scope of change is correct for the PR.

## Solution

Updates examples to use the Improved Spawning API merged in
https://github.com/bevyengine/bevy/pull/17521

## Testing

- Did you test these changes? If so, how?
- Opened the examples before and after and verified the same behavior
was observed. I did this on Ubuntu 24.04.2 LTS using `--features
wayland`.
- Are there any parts that need more testing?
- Other OS's and features can't hurt, but this is such a small change it
shouldn't be a problem.
- How can other people (reviewers) test your changes? Is there anything
specific they need to know?
  - Run the examples yourself with and without these changes.
- If relevant, what platforms did you test these changes on, and are
there any important ones you can't test?
  - see above

---

## Showcase

n/a

## Migration Guide

n/a

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

examples/app/log_layers_ecs.rs

@@ -150,13 +150,16 @@ fn print_logs(
 
     commands.entity(root_entity).with_children(|child| {
         for event in events.read() {
-            child.spawn(Text::default()).with_children(|child| {
-                child.spawn((
-                    TextSpan::new(format!("{:5} ", event.level)),
-                    TextColor(level_color(&event.level)),
-                ));
-                child.spawn(TextSpan::new(&event.message));
-            });
+            child.spawn((
+                Text::default(),
+                children![
+                    (
+                        TextSpan::new(format!("{:5} ", event.level)),
+                        TextColor(level_color(&event.level)),
+                    ),
+                    TextSpan::new(&event.message),
+                ],
+            ));
         }
     });
 }