Remove 'include-zero' from cumsum example (ch. 6)

We currently have this: 

`top=cumsum("passengers", include_zero=True),`

But it might be more intuitive to start with the sum in January at January's value (instead of zero):

![image](https://github.com/bokeh/tutorial/assets/39711796/54853449-6f03-4b72-a8d9-aef9ea6cf99e)

