Use "start" rather than "rootIndex"

Author: przemekwitek

x-pack/plugin/esql/compute/src/main/java/org/elasticsearch/compute/data/sort/BytesRefBucketedSort.java

@@ -210,9 +210,9 @@ public Block toBlock(BlockFactory blockFactory, IntVector selected) {
 
                 // If we are in the gathering mode, we need to heapify before sorting.
                 if (common.inHeapMode(bucket) == false) {
-                    heapify(rootIndex, (int) size);
+                    heapify(start, (int) size);
                 }
-                heapSort(rootIndex, (int) size);
+                heapSort(start, (int) size);
 
                 builder.beginPositionEntry();
                 for (int i = 0; i < size; i++) {