Add upgrade cluster to mkdocs 261 (#928)

* Update mkdocs.yml (#923)

* Update mkdocs.yml

mkdocs.yml

@@ -463,7 +463,7 @@ nav:
         - Deploy clusters with Helm: nebula-operator/3.deploy-nebula-graph-cluster/3.2create-cluster-with-helm.md
       - Configure clusters:
         - Custom configuration parameters for a Nebula Graph cluster: nebula-operator/8.custom-cluster-configurations/8.1.custom-conf-parameter.md
-        - Reclaim PV: nebula-operator/8.custom-cluster-configurations/8.2.pv-reclaim.md 
+        - Reclaim PVs: nebula-operator/8.custom-cluster-configurations/8.2.pv-reclaim.md 
         - Balance storage data after scaling out: nebula-operator/8.custom-cluster-configurations/8.3.balance-data-when-scaling-storage.md
       - Upgrade Nebula Graph clusters: nebula-operator/9.upgrade-nebula-cluster.md
       - Connect to Nebula Graph databases: nebula-operator/4.connect-to-nebula-graph-service.md