Update README.md

Author: assassint2017

README.md

@@ -28,7 +28,7 @@
 19. [Remove Nth Node From End of List](https://github.com/assassint2017/leetcode-algorithm/tree/master/Remove%20Nth%20Node%20From%20End%20of%20List)
 20. [Valid Parentheses](https://github.com/assassint2017/leetcode-algorithm/tree/master/Valid%20Parentheses)
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-21. [Merge Two Sorted Lists]()
+21. [Merge Two Sorted Lists](https://github.com/assassint2017/leetcode-algorithm/tree/master/Merge%20Two%20Sorted%20Lists)
 22. [Generate Parentheses]()
 23. [Merge k Sorted Lists]()
 24. [Swap Nodes in Pairs]()