Updated client library documentation URLs. (#7307)

Previously, the URLs would redirect using JavaScript, which would either be slow or not work at all (in case JavaScript is disabled on the browser)

Author: pravindahal

README.rst

@@ -11,7 +11,7 @@ common helpers used by all Google API clients. For more information, see the
    :target: https://pypi.org/project/google-api_core/
 .. |versions| image:: https://img.shields.io/pypi/pyversions/google-api_core.svg
    :target: https://pypi.org/project/google-api_core/
-.. _documentation: https://googlecloudplatform.github.io/google-cloud-python/latest/core/
+.. _documentation: https://googleapis.github.io/google-cloud-python/latest/core/
 
 
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