fix ball altitude not being displayed anymore from basic_sim3 example…

[Vrixyz]

• Follow up from report from Testing documentation examples #99 (review)

[rlidwka]

It should be exact copy of 2d variant:

fn print_ball_altitude(positions: Query<&Transform, With<RigidBody>>) {
    for transform in positions.iter() {
        println!("Ball altitude: {}", transform.translation.y);
    }
}

no need for muts