Fix link

Author: richvdh

CONTRIBUTING.rst

@@ -117,7 +117,7 @@ license - in our case, this is Apache Software License v2 (see LICENSE).
 In order to have a concrete record that your contribution is intentional
 and you agree to license it under the same terms as the project's license, we've adopted the
 same lightweight approach used by the `Linux Kernel <https://www.kernel.org/doc/html/latest/process/submitting-patches.html>`_,
-`Docker <https://github.com/docker/docker/blob/master/CONTRIBUTING.md`_, and many other
+`Docker <https://github.com/docker/docker/blob/master/CONTRIBUTING.md>`_, and many other
 projects: the `Developer Certificate of Origin <http://developercertificate.org/>`_
 (DCO). This is a simple declaration that you wrote
 the contribution or otherwise have the right to contribute it to Matrix::