use Mul<f32> to double the value of Vec3 (bevyengine#6607)

improve the example code

examples/ecs/iter_combinations.rs

@@ -175,8 +175,7 @@ fn integrate(mut query: Query<(&mut Acceleration, &mut Transform, &mut LastPos)>
         // verlet integration
         // x(t+dt) = 2x(t) - x(t-dt) + a(t)dt^2 + O(dt^4)
 
-        let new_pos =
-            transform.translation + transform.translation - last_pos.0 + acceleration.0 * dt_sq;
+        let new_pos = transform.translation * 2.0 - last_pos.0 + acceleration.0 * dt_sq;
         acceleration.0 = Vec3::ZERO;
         last_pos.0 = transform.translation;
         transform.translation = new_pos;