Document bevy_scene requirement for example

[Eterniance]

Objective

Add the bevy_scene feature requirement in the ColliderConstructorHierarchy documentation example. Fixes #874.

[Jondolf]

So I think the relevant part here is that the type itself requires the bevy_scene feature for it to work on scenes, not that this specific example requires bevy_scene.

I pushed a commit to tweak the docs a bit and mention the bevy_scene requirement earlier on in the docs, in the section where it describes how the component interacts with SceneRoot :)