Fix comment in example (#15096)

Commit 65252bb (Consistently use `PI` to specify angles in examples.
(#5825)) changed from using push_children to add_child without adjusting
the comment

# Objective

- Fix the comments to align with the code

Co-authored-by: Alexander Stein <alexander.stein@mailbox.org>

examples/ecs/hierarchy.rs

@@ -40,7 +40,7 @@ fn setup(mut commands: Commands, asset_server: Res<AssetServer>) {
  // Store parent entity for next sections
  .id();
 
- // Another way is to use the push_children function to add children after the parent
+ // Another way is to use the add_child function to add children after the parent
  // entity has already been spawned.
  let child = commands
  .spawn(SpriteBundle {