Do not use outdated schedule_interval in tutorial dags (apache#50947)

Author: amoghrajesh

airflow-core/docs/tutorial/taskflow.rst

@@ -159,7 +159,7 @@ Here's what the same DAG might have looked like using the traditional approach:
 
    with DAG(
        dag_id="legacy_etl_pipeline",
-       schedule_interval=None,
+       schedule=None,
        start_date=pendulum.datetime(2021, 1, 1, tz="UTC"),
        catchup=False,
        tags=["example"],