replace one-country pandemic planning scenario figure with multilateral source

> I think this is a useful diagram but would suggest preferring a diagram from a multilateral source, e.g. WHO, PAHO, over an assessment from a single national agency. The title needs to be more clear that these are influenza scenarios, although it is included in the caption.

_Originally posted by @pratikunterwegs in https://github.com/epiverse-trace/tutorials-middle/pull/1#discussion_r1502500504_