Updating wordpress tutorial for helm 3

[slow-J]

https://github.com/heptio/aws-quickstart/blob/master/docs/tutorial-wordpress.md
In Helm 3, tiller has been removed but is now not necessary, all the tiller section can be skipped.

During the 3. Install Wordpress section you need to now run:
helm repo add stable https://kubernetes-charts.storage.googleapis.com/ in order to have access to the stable/wordpress repo, before the command:
helm install --namespace varmywordpress wordpress stable/wordpress
(name parameter is no longer optional).