Change master in links to HEAD

Author: wooorm

readme.md

@@ -300,11 +300,11 @@ abide by its terms.
 
 [author]: https://wooorm.com
 
-[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
 
@@ -336,6 +336,6 @@ abide by its terms.
 
 [handler]: #optionshandlers
 
-[handlers]: https://github.com/syntax-tree/hast-util-to-mdast/tree/master/lib/handlers
+[handlers]: https://github.com/syntax-tree/hast-util-to-mdast/tree/main/lib/handlers
 
 [spec]: https://html.spec.whatwg.org/#paragraphs