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

## Describe the issue

The `FullscreenMaterial` API Changes [example](https://bevy.org/learn/migration-guides/0-18-to-0-19/#fullscreenmaterial-api-changes) lists trait functions `run_in`, `run_after`, and `run_before`, which do no appear in the `bevy@0.18` `FullscreenMaterial` trait:

```rust
// 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 {
None
}
fn run_before() -> Option {
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:

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

Start at the FullscreenMaterial API Changes example in the linked 0.18-to-0.19 migration guide and compare its 0.18 snippet with the 0.18 FullscreenMaterial API described in the issue. Update the example so it uses the documented 0.18 methods, then verify the migration guide renders correctly and no longer lists the incorrect functions.

Written by the indexing model from the issue text.