update gltf example to use type-safe GltfAssetLabel::Scene (#14218)

# Objective

update the `load_gltf_extras.rs` example to the newest bevy api

## Solution

uses the new type-safe code for loading the scene #0 from the gltf
instead of a path suffix

## Testing

the example runs as expected

examples/3d/load_gltf_extras.rs

@@ -29,9 +29,11 @@ fn setup(mut commands: Commands, asset_server: Res<AssetServer>) {
         },
         ..default()
     });
+
     // a barebones scene containing one of each gltf_extra type
     commands.spawn(SceneBundle {
-        scene: asset_server.load("models/extras/gltf_extras.glb#Scene0"),
+        scene: asset_server
+            .load(GltfAssetLabel::Scene(0).from_asset("models/extras/gltf_extras.glb")),
         ..default()
     });