[AIRFLOW-5182] remove unused incorrect import (#5867)

docs/kubernetes.rst

@@ -33,7 +33,6 @@ any type of executor.
 
 .. code:: python
 
-    from airflow.contrib.operators import KubernetesOperator
     from airflow.contrib.operators.kubernetes_pod_operator import KubernetesPodOperator
     from airflow.contrib.kubernetes.secret import Secret
     from airflow.contrib.kubernetes.volume import Volume