Fix: off-by-one in the boid count enumeration

`enumerate()` begins counting at 0, so I was dividing the sum by one
less than the count of boids. Thus the over-expanded result.

I should do unit testing.

I should rewrite this function to take the vector directly. I *know*
that the input is a vector at all times. Mmh.

src/birdoids_plugin.rs

@@ -226,8 +226,8 @@ pub(crate) fn center_of_boids(points: impl Iterator<Item = Vec2>) -> Option<Vec2
             // add running sum & new point for new running sum
             (idx, sum + point)
         });
+    let avg = sum / ((len + 1) as f32);
     
-    let avg = sum / (len as f32);
     Some(avg)
 }