Fix: off-by-one error in alignment averaging

The "length" is actually the `enumerate()` index, which is one less than
the item count. The previous version was not using the average, and may
have been deviding by 0 for boids with exactly one neighbor.

src/birdoids/mod.rs

@@ -251,7 +251,7 @@ fn alignment(
 
         // Skip to next boid if the current one has no neighbors.
         let avg = if len > 0 {
-            sum / (len as f32)
+            sum / ((len + 1) as f32)
         } else {
             continue;
         };