fix: link to leetcode added

Author: mjk22071998

src/main/java/com/thealgorithms/datastructures/trees/BinaryTreeToString.java

@@ -1,6 +1,9 @@
 package com.thealgorithms.datastructures.trees;
 
 /**
+ * Leetcode 606: Construct String from Binary Tree:
+ * https://leetcode.com/problems/construct-string-from-binary-tree/
+ *
  * Utility class to convert a {@link BinaryTree} into its string representation.
  * <p>
  * The conversion follows a preorder traversal pattern (root → left → right)