[hold] Moves the countdown method to main actor

[icanzilb]

In one part of chapter 4 the countdown uses a Timer and in iOS16 sometimes the code ends up off the main thread and timer doesn't work, this PR annotates the method on the main actor

[jrturton]

I think the general advice is that an observable object should be annotated as main actor at the class level, there are now multiple methods in here that are main actor annotated, should we switch it over?

[icanzilb]

@jrturton this might make sense indeed, I'm just thinking that we'll need to likely change text & code across 3 chapters to spread these changes. You think we should do that or?

[jrturton]

If that's the way we're supposed to describe these objects, then yes - @freak4pc do you have an opinion?

[freak4pc]

Marin mentioned you resolved this, closing :)