FullscreenMaterial API Changes example is incorrect in migration guide 0.18 -> 0.19

[DavidHospital]

Describe the issue

The FullscreenMaterial API Changes example lists trait functions run_in, run_after, and run_before, which do no appear in the bevy@0.18 FullscreenMaterial trait:

// 0.18
impl FullscreenMaterial for FullscreenEffect {
    fn fragment_shader() -> ShaderRef {
        "shaders/fullscreen_effect.wgsl".into()
    }
    fn run_in() -> impl SystemSet {
        Core3dSystems::PostProcess
    }
    fn run_after() -> Option<Core3dSystems> {
        None
    }
    fn run_before() -> Option<Core3dSystems> {
        None
    }
}

I am not entirely sure where these functions came from, as this feature was first implemented in 0.18, and neither the 0.18 or 0.19 docs reference these functions anywhere, to my knowledge.

Proposed Solution

Replace the 0.18 section of the example with something like:

// 0.18
impl FullscreenMaterial for FullscreenEffect {
    fn fragment_shader() -> ShaderRef {
        "shaders/fullscreen_effect.wgsl".into()
    }
    fn node_edges() -> Vec<InternedRenderLabel> {
        vec![
            Node3d::Tonemapping.intern(),
            Self::node_label().intern(),
            Node3d::EndMainPassPostProcessing.intern(),
        ]
    }
}