Update upgrade.md

Author: kandelira

docs/admin/runai-setup/self-hosted/k8s/upgrade.md

@@ -25,6 +25,8 @@ title: Upgrade self-hosted Kubernetes installation
 Before upgrading the control plane, run:
 
 ``` bash
+POSTGRES_PV=$(kubectl get pvc pvc-postgresql -n runai-backend -o jsonpath='{.spec.volumeName}')
+THANOS_PV=$(kubectl get pvc pvc-thanos-receive -n runai-backend -o jsonpath='{.spec.volumeName}')
 kubectl patch pv $POSTGRES_PV $THANOS_PV -p '{"spec":{"persistentVolumeReclaimPolicy":"Retain"}}'
 ```