Use smooth_nudge in 2d_top_down_camera example (bevyengine#13907)

# Objective


[`StableInterpolate`](https://dev-docs.bevyengine.org/bevy/math/trait.StableInterpolate.html)
was introduced after this example was written (or more accurately, the
two PRs were merged on the same day and developed in parallel).

The example used `Vec3::lerp` where I believe it is now preferred to use
`smooth_nudge`.

## Solution

Its not entirely clear to me whether `StableInterpolate` should be
preferred in this scenario, although it seems likely. So I figured a PR
to make the change would either result in it being merged or denied with
a reason.

## Testing

```
cargo run --example 2d_top_down_camera
```

Co-authored-by: François Mockers <mockersf@gmail.com>

examples/camera/2d_top_down_camera.rs

@@ -17,8 +17,8 @@ use bevy::sprite::{MaterialMesh2dBundle, Mesh2dHandle};
 /// Player movement speed factor.
 const PLAYER_SPEED: f32 = 100.;
 
-/// Camera lerp factor.
-const CAM_LERP_FACTOR: f32 = 2.;
+/// How quickly should the camera snap to the desired location.
+const CAMERA_DECAY_RATE: f32 = 2.;
 
 #[derive(Component)]
 struct Player;
@@ -103,14 +103,11 @@ fn update_camera(
  let Vec3 { x, y, .. } = player.translation;
  let direction = Vec3::new(x, y, camera.translation.z);
 
- // Applies a smooth effect to camera movement using interpolation between
- // the camera position and the player position on the x and y axes.
- // Here we use the in-game time, to get the elapsed time (in seconds)
- // since the previous update. This avoids jittery movement when tracking
- // the player.
- camera.translation = camera
+ // Applies a smooth effect to camera movement using stable interpolation
+ // between the camera position and the player position on the x and y axes.
+ camera
  .translation
- .lerp(direction, time.delta_seconds() * CAM_LERP_FACTOR);
+ .smooth_nudge(&direction, CAMERA_DECAY_RATE, time.delta_seconds());
 }
 
 /// Update the player position with keyboard inputs.