Fix shader_prepass example (#15719)

Simple fix for another missed piece in the port to required components

examples/shader/shader_prepass.rs

@@ -212,7 +212,7 @@ impl Material for PrepassOutputMaterial {
 fn toggle_prepass_view(
  mut prepass_view: Local<u32>,
  keycode: Res<ButtonInput<KeyCode>>,
- material_handle: Query<&Handle<PrepassOutputMaterial>>,
+ material_handle: Query<&MeshMaterial3d<PrepassOutputMaterial>>,
  mut materials: ResMut<Assets<PrepassOutputMaterial>>,
  mut text: Query<&mut Text>,
 ) {