Update docs authentication link to current URL. (#9187)

Closes #9183.

Author: tseaver

README.rst

@@ -185,7 +185,7 @@ Check out the `Authentication section`_ in our documentation to learn more.
 You may also find the `authentication document`_ shared by all the
 ``google-cloud-*`` libraries to be helpful.
 
-.. _Authentication section: https://google-cloud-python.readthedocs.io/en/latest/core/auth.html
+.. _Authentication section: https://googleapis.dev/python/google-api-core/latest/auth.html
 .. _authentication document: https://github.com/googleapis/google-cloud-common/tree/master/authentication
 
 Contributing