fix: typo

Author: lucifer

Diff for: thinkings/heap.md

@@ -781,7 +781,7 @@ public class Heap {
     }
 
     public void buildHeap() {
-        for (int i = size >> 1; i >= 0; i--) {
+        for (int i = size >> 1; i > 0; i--) {
             shiftDown(i);
         }
     }