fix schedule_interval cron expression

The cron expression currently used to specify the `schedule_interval` will lead to a run at every hour (not just hour 0). See [crontab.guru's description](https://crontab.guru/#0_*_*_*_*) for details.
For a daily run, the correct expression is `0 0 * * *`

README.md

@@ -188,7 +188,7 @@ with DAG('airflow_tutorial_v01',
          ) as dag:
 ```
 
-With `schedule_interval='0 * * * *'` we've specified a run at every hour 0; the DAG will run each day at 00:00.
+With `schedule_interval='0 0 * * *'` we've specified a run at every hour 0; the DAG will run each day at 00:00.
 See [crontab.guru](https://crontab.guru/#0_*_*_*_*) for help deciphering cron schedule expressions.
 Alternatively, you can use strings like `'@daily'` and `'@hourly'`.