Fix Fork link

Author: oleksis

CONTRIBUTING.rst

@@ -129,7 +129,7 @@ First time setup
 .. _username: https://docs.github.com/en/github/using-git/setting-your-username-in-git
 .. _email: https://docs.github.com/en/github/setting-up-and-managing-your-github-user-account/setting-your-commit-email-address
 .. _GitHub account: https://github.com/join
-.. _Fork: https://github.com/pallets/jinja/fork
+.. _Fork: https://github.com/pallets/flask/fork
 .. _Clone: https://docs.github.com/en/github/getting-started-with-github/fork-a-repo#step-2-create-a-local-clone-of-your-fork