[AIRFLOW-XXX] Update kubernetes.rst docs (#3875)

Update kubernetes.rst with correct KubernetesPodOperator inputs
for the volumes.

docs/kubernetes.rst

@@ -126,7 +126,7 @@ Kubernetes Operator
                               arguments=["echo", "10"],
                               labels={"foo": "bar"},
                               secrets=[secret_file,secret_env]
-                              volume=[volume],
+                              volumes=[volume],
                               volume_mounts=[volume_mount]
                               name="test",
                               task_id="task",