Update tutorial docs -> function as custom payload

[VictorWinberg]

When we give a function as the custom payload, Hyperapp considers it a payload filter and passes the default payload through it, providing the returned value as payload to the action.

oninput: [SetFilter, (event) => event.target.value], 

This does not work since function is not executed just returned.

[VictorWinberg]

Duplicate of #965