fix(doc): replace the legacy helm delete command by helm uninstall (datastax#104)

Refer: https://helm.sh/docs/helm/helm_delete/

Author: ericsyh

docs/kaap.md

@@ -26,7 +26,7 @@ kubectl wait pulsar pulsar-cluster --for condition=Ready=True --timeout=240s
 
 Uninstall the operator and the PulsarCluster
 ```
-helm delete kaap
+helm uninstall kaap
 ```