Fix error in README.md (#508)

* Fix error in README.md

In the README.md file, there was an error in the kubectl delete command with an extra "s".

* Add space in README.md

With my change, i miss to add space in line.

README.md

@@ -237,7 +237,7 @@ The Pods will be deleted in a few minutes.
 > ```shell
 > kubectl delete -f risingwave.yaml   # Delete all resources defined in the risingwave.yaml that you used above.
 > kubectl delete pvc -l app=etcd      # Delete the PVCs of etcd.
-> kubectl deletes pvc -l app=minio    # Delete the PVCs of MinIO.
+> kubectl delete pvc -l app=minio     # Delete the PVCs of MinIO.
 > ```
 
 ### Customize the RisingWave cluster