fixing Cassandra shutdown example to avoid data corruption

examples/storage/cassandra/cassandra-statefulset.yaml

@@ -34,6 +34,10 @@ spec:
           capabilities:
             add:
               - IPC_LOCK
+        lifecycle:
+          preStop:
+            exec:
+              command: ["/bin/sh", "-c", "PID=$(pidof java) && kill $PID && while ps -p $PID > /dev/null; do sleep 1; done"]
         env:
           - name: MAX_HEAP_SIZE
             value: 512M