UI/text Example - Clarity Improvements (#16302)

# Objective

- Fixes #16292 

## Solution

- Renames the `ColorText` marker to `AnimatedText`, which is more
distinct from the `TextColor` Bevy component.
- Changes the comment language from `A unit struct` to `Marker struct`
for better consistency with other Bevy docs.

## Testing

- Locally, example still runs just fine

examples/ui/text.rs

@@ -17,13 +17,13 @@ fn main() {
         .run();
 }
 
-// A unit struct to help identify the FPS UI component, since there may be many Text components
+// Marker struct to help identify the FPS UI component, since there may be many Text components
 #[derive(Component)]
 struct FpsText;
 
-// A unit struct to help identify the color-changing Text component
+// Marker struct to help identify the color-changing Text component
 #[derive(Component)]
-struct ColorText;
+struct AnimatedText;
 
 fn setup(mut commands: Commands, asset_server: Res<AssetServer>) {
     // UI camera
@@ -47,7 +47,7 @@ fn setup(mut commands: Commands, asset_server: Res<AssetServer>) {
             right: Val::Px(5.0),
             ..default()
         },
-        ColorText,
+        AnimatedText,
     ));
 
     // Text with multiple sections
@@ -116,7 +116,7 @@ fn setup(mut commands: Commands, asset_server: Res<AssetServer>) {
     ));
 }
 
-fn text_color_system(time: Res<Time>, mut query: Query<&mut TextColor, With<ColorText>>) {
+fn text_color_system(time: Res<Time>, mut query: Query<&mut TextColor, With<AnimatedText>>) {
     for mut text_color in &mut query {
         let seconds = time.elapsed_secs();