Best Practice for using variable in timetable seems obsolete

### What do you see as an issue?

hello, it seems that the example provided in this page [https://airflow.apache.org/docs/apache-airflow/stable/best-practices.html#timetables ](https://airflow.apache.org/docs/apache-airflow/stable/best-practices.html#timetables )
is not valid anymore, not only it doesn't works but it also seems to crash dag processor

### Solving the problem

If this example is not valid it should be removed (and maybe dag processor should not crash and provide import issue on UI)

### Anything else

_No response_

### Are you willing to submit PR?

- [ ] Yes I am willing to submit a PR!

### Code of Conduct

- [x] I agree to follow this project's [Code of Conduct](https://github.com/apache/airflow/blob/main/CODE_OF_CONDUCT.md)