Use string in Ebola vignette and pass R CMD CHECK on Windows latest

[athowes]

@seabbs for some reason (!?) putting string = "lognormal" causes failure only in the Ebola vignette, and only on Windows latest. Going to make a new issue to resolve this but in the meantime so that we don't have failing tests on main / the rest of the branches I'm in favour of putting this in

Originally posted by @athowes in #334 (comment)

(This is very confusing to me, and hard to debug...)

[athowes]

• Put print statements
• Try using a Windows machine
• Confirm that other string vignettes work: confirmed, yes they do

[athowes]

The error is:

Error: Error: processing vignette 'ebola.Rmd' failed with diagnostics:
missing value where TRUE/FALSE needed

This doesn't really help but nonetheless.

[athowes]

With PR #449 we reverted to using lognormal() hence we no longer have this issue (though we didn't solve it as is).