Change master in links to HEAD

Author: wooorm

readme.md

@@ -450,11 +450,11 @@ The initial release of this project was authored by **[@wooorm][author]**.
 
 [health]: https://github.com/syntax-tree/.github
 
-[contributing]: https://github.com/syntax-tree/.github/blob/master/contributing.md
+[contributing]: https://github.com/syntax-tree/.github/blob/HEAD/contributing.md
 
-[support]: https://github.com/syntax-tree/.github/blob/master/support.md
+[support]: https://github.com/syntax-tree/.github/blob/HEAD/support.md
 
-[coc]: https://github.com/syntax-tree/.github/blob/master/code-of-conduct.md
+[coc]: https://github.com/syntax-tree/.github/blob/HEAD/code-of-conduct.md
 
 [awesome]: https://github.com/syntax-tree/awesome-syntax-tree