s/gcloud-common/google-cloud-common/g (#4180)

The gcloud-common repo moved to
https://github.com/GoogleCloudPlatform/google-cloud-common

Author: tswast

README.rst

@@ -27,7 +27,7 @@ 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 document: https://github.com/GoogleCloudPlatform/gcloud-common/tree/master/authentication
+.. _authentication document: https://github.com/GoogleCloudPlatform/google-cloud-common/tree/master/authentication
 
 Using the API
 -------------