Fix: apply steering input properly

The ship in Asteroids isn't expected to spin up while holding a steering
direction, but that's exactly what I just made it do.

Fix that problem by assigning, not accumulating, the angular velocity
for the ship.

src/ship.rs

@@ -1,5 +1,6 @@
 use crate::{
-    physics::{Velocity, Wrapping}, AngularVelocity, GameAssets
+    AngularVelocity, GameAssets,
+    physics::{Velocity, Wrapping},
 };
 
 use bevy::prelude::*;