docs: update URLs in the helm charts writeup

https://docs.voltha.org/master/voltha-helm-charts/README.html

• Helm Charts to Deploy VOLTHA 2.x

More information and documentation can be found in the voltha docs.

We may have a subtle caching problem in play.  The initial click on "voltha docs" landed on a page hosted on https://opennetworking.org.  Subsequent clicks after the initial attempt finally land on a docs.voltha page:

    https://docs.voltha.org/master/index.html

Wait for cached entries to expire then see if this behavior is repeatable.