redraw figure to use presymptomatic instead of asymptomatic

> Strictly speaking, I'd define this as 'presymptomatic' and 'symptomatic', with asymptomatic reserved for those who never develop symptoms (e.g. https://bmcinfectdis.biomedcentral.com/articles/10.1186/s12879-022-07440-0). But perhaps an edit for a later version – if needed, we could also make new schematics using some plots I've already made for various COVID presentations.

_Originally posted by @adamkucharski in https://github.com/epiverse-trace/tutorials-early/pull/19#discussion_r1538990783_