Add index of LeetCode0540.java to docs

Author: dunso

docs/_posts/2017-12-03-String.md

@@ -36,6 +36,7 @@ icon: file-alt
 |0521|[Longest Uncommon Subsequence I](https://leetcode.com/problems/longest-uncommon-subsequence-i/)|[Java](https://algorithm.dun.so/leetcode-521/)|-|-|-|Easy|String|
 |0522|[Longest Uncommon Subsequence II](https://leetcode.com/problems/longest-uncommon-subsequence-ii/)|[Java](https://algorithm.dun.so/leetcode-522/)|-|-|-|Medium|String|
 |0539|[Minimum Time Difference](https://leetcode.com/problems/minimum-time-difference/)|[Java](https://algorithm.dun.so/leetcode-539/)|-|-|-|Medium|String|
+|0540|[Single Element in a Sorted Array](https://leetcode.com/problems/single-element-in-a-sorted-array/)|[Java](https://algorithm.dun.so/leetcode-540/)|-|-|-|Medium|String|
 |0591|[Tag Validator](https://leetcode.com/problems/tag-validator)| [Java](https://algorithm.dun.so/leetcode-591/)  | O(n) |19MS| O(n)  |  Hard |String|
 |0567|[Permutation in String](https://leetcode.com/problems/permutation-in-string)| [Java](https://algorithm.dun.so/leetcode-567/)  | O(n) |-| O(n)  |  Medium |String|