Update README.md

Author: zewenw

solution/README.md

@@ -19,7 +19,7 @@
 |  0006  |  [N 字形变换](/solution/0000-0099/0006.Zigzag%20Conversion/README.md)  |  `字符串`  |  中等  |  done  |
 |  0007  |  [整数反转](/solution/0000-0099/0007.Reverse%20Integer/README.md)  |  `数学`  |  中等  | done   |
 |  0008  |  [字符串转换整数 (atoi)](/solution/0000-0099/0008.String%20to%20Integer%20%28atoi%29/README.md)  |  `字符串`  |  中等  |  done  |
-|  0009  |  [回文数](/solution/0000-0099/0009.Palindrome%20Number/README.md)  |  `数学`  |  简单  |    |
+|  0009  |  [回文数](/solution/0000-0099/0009.Palindrome%20Number/README.md)  |  `数学`  |  简单  |   done |
 |  0010  |  [正则表达式匹配](/solution/0000-0099/0010.Regular%20Expression%20Matching/README.md)  |  `递归`,`字符串`,`动态规划`  |  困难  |    |
 |  0011  |  [盛最多水的容器](/solution/0000-0099/0011.Container%20With%20Most%20Water/README.md)  |  `贪心`,`数组`,`双指针`  |  中等  | done   |
 |  0012  |  [整数转罗马数字](/solution/0000-0099/0012.Integer%20to%20Roman/README.md)  |  `哈希表`,`数学`,`字符串`  |  中等  |    |