Link heading of README to Leetcode page

Author: fteem

20. Valid Parentheses/README.md

@@ -1,9 +1,7 @@
-# Valid Parentheses
+# [Valid Parentheses](https://leetcode.com/problems/valid-parentheses/description/)
 
 ## Description
 
 Given a string containing just the characters `(`, `)`, `{`, `}`, `[` and `]`, determine if the input string is valid.
 
 The brackets must close in the correct order, `()` and `()[]{}` are all valid but `(]` and `([)]` are not.
-
-[Source](https://leetcode.com/problems/valid-parentheses/description/)