[Doc] Prefer relative path in CONTRIBUTING.md

This patch by nobu (Nobuyoshi Nakada), thank you!

ref: #4266 (comment)

Co-authored-by: Nobuyoshi Nakada <nobu@ruby-lang.org>

CONTRIBUTING.md

@@ -1,5 +1,5 @@
 Please see the [official issue tracker], [doc/contributing.rdoc] and wiki [HowToContribute].
 
 [official issue tracker]: https://bugs.ruby-lang.org
-[doc/contributing.rdoc]: https://github.com/ruby/ruby/blob/master/doc/contributing.rdoc
+[doc/contributing.rdoc]: doc/contributing.rdoc
 [HowToContribute]: https://bugs.ruby-lang.org/projects/ruby/wiki/HowToContribute