Fix: alignment should align to vel, not impulse

I need to apply an impulse to match the velocities, but the previous
version was trying to match the forces.

src/birdoids/mod.rs

@@ -256,7 +256,8 @@ fn alignment(
             continue;
         };
 
-        force.0 = (avg.extend(0.0) - force.0) * ALIGNMENT_FACTOR;
+        let boid_vel = boid_velocities.get(this_entt).unwrap();
+        force.0 += (avg.extend(0.0) - boid_vel.0) * ALIGNMENT_FACTOR;
     }
 }