clarify the expected identity between cfr_static and cfr_rolling at the end of the outbreak

from translation review by @Karim-Mane

Edit we can do:

> they will tent to converge to the same value, but we only reach that point after observing all the expected outcomes of the observed cases at the day of analysis.
> if at any point we say "the same" we mean that the uncertainty intervals of both are overlapping

In
https://github.com/epiverse-trace/tutorials-middle/blob/a7cd4328ab3a594f79f98542ccf67452659d8817/episodes/severity-static.Rmd#L487

repeated in
https://github.com/epiverse-trace/tutorials-middle/blob/a7cd4328ab3a594f79f98542ccf67452659d8817/episodes/severity-static.Rmd#L841