docs: update the helm install instructions

[ciarams87]

Proposed changes

Update the helm readme with the correct 'stable' repo. I didn't update the patch version - I can do that in a follow up commit/ MR if required.

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto master
• I will ensure my PR is targeting the master branch and pulling from my branch from my own fork