Fix: Helm uninstall documentation to delete PVCs

[mridulji]

What this PR does / Why we need it: open-match requires redis to store the data and perform functional requirements on it. Redis is deployed via it's helm chart which creates PVCs. Helm command for uninstalling open-match release is not authorised to delete components created by another chart i.e. redis. So to fix this, we need to delete open-match namespace using kubectl command

Which issue(s) this PR fixes:

Closes googleforgames/open-match/issues/1477

Special notes for your reviewer:

• Updated the url of unity.com because it was observed that cloud-build failed few times in other PR.