Fix index for batch remainder (#21)

src/prefecto/concurrency.py

@@ -89,7 +89,7 @@ def _make_batches(self, **params) -> list[dict[str, list[Any]]]:
         if length % self.size != 0:
             batch = {p: [] for p in parameters}
             for p in parameters:
-                batch[p] = params[p][(i + 1) * self.size :]
+                batch[p] = params[p][(length // self.size) * self.size :]
             batches.append(batch)
 
         return batches