fix unstable confidence intervals in {EpiNow2} output

output available at https://epiverse-trace.github.io/tutorials-middle/delays-functions.html#use-an-incubation-period-for-covid-19-to-estimate-rt

sample figure output pasted in reply comments

> The CI on the plot `fig/delays-functions-rendered-unnamed-chunk-17-1.png` have changed quite dramatically. Is this expected?

> https://github.com/epiverse-trace/tutorials-middle/commit/b2ec9db790249cfe07b9437c9457e8f70444ab12#diff-c8ccb4af4f83edac20865ecd44127afcd1986c35efb7c98399c6684abccc0897

_Originally posted by @Bisaloo in https://github.com/epiverse-trace/tutorials-middle/pull/163#pullrequestreview-2731760571_