Change master in links to HEAD

Author: wooorm

readme.md

@@ -140,11 +140,11 @@ abide by its terms.
 
 [author]: https://keith.mcknig.ht
 
-[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
 
 [tree]: https://github.com/syntax-tree/unist#tree