Update App Engine link

Point to an authoritative site for Flask on App Engine that is maintained.

docs/deploying/index.rst

@@ -20,7 +20,7 @@ Hosted options
 - `Deploying Flask on Heroku <https://devcenter.heroku.com/articles/getting-started-with-python>`_
 - `Deploying Flask on OpenShift <https://developers.openshift.com/en/python-flask.html>`_
 - `Deploying Flask on Webfaction <http://flask.pocoo.org/snippets/65/>`_
-- `Deploying Flask on Google App Engine <https://github.com/kamalgill/flask-appengine-template>`_
+- `Deploying Flask on Google App Engine <https://cloud.google.com/appengine/docs/standard/python/getting-started/python-standard-env>`_
 - `Deploying Flask on AWS Elastic Beanstalk <https://docs.aws.amazon.com/elasticbeanstalk/latest/dg/create-deploy-python-flask.html>`_
 - `Sharing your Localhost Server with Localtunnel <http://flask.pocoo.org/snippets/89/>`_
 - `Deploying on Azure (IIS) <https://azure.microsoft.com/documentation/articles/web-sites-python-configure/>`_