doc(asset): fix asset trait example (#9105)

# Objective

Fix the example code for the `Asset` trait.

## Solution

Add `TypePath` trait on `CustomAsset`.
Add a static check.

crates/bevy_asset/src/loader.rs

@@ -34,14 +34,16 @@ pub trait AssetLoader: Send + Sync + 'static {
 /// and scripts. In Bevy, an asset is any struct that has an unique type id, as shown below:
 ///
 /// ```rust
-/// use bevy_reflect::TypeUuid;
+/// use bevy_reflect::{TypePath, TypeUuid};
 /// use serde::Deserialize;
 ///
-/// #[derive(Debug, Deserialize, TypeUuid)]
+/// #[derive(Debug, Deserialize, TypeUuid, TypePath)]
 /// #[uuid = "39cadc56-aa9c-4543-8640-a018b74b5052"]
 /// pub struct CustomAsset {
 /// pub value: i32,
 /// }
+/// # fn is_asset<T: bevy_asset::Asset>() {}
+/// # is_asset::<CustomAsset>();
 /// ```
 ///
 /// See the `assets/custom_asset.rs` example in the repository for more details.