Minor doc fixes

[mattwarkentin]

Hi @topepo,

Just a few minor fixes I noticed when playing around with this package. Hope it is helpful.

For the list -> vector change, it turns out categories can't be a list because it fails check_unit_range(), specifically:

desirability2/R/checks.R

Line 17 in 564fc8a

if (!is.vector(x) || !is.numeric(x)) {

[mattwarkentin]

Pretty confident that single Windows failure is unrelated to this PR...😬

[topepo]

Thanks!