Convert checks for 9.6.3 into (good) list examples

[Tazinho]

Exercises 9.6.3 number 3 is intended to work on lists (which it does), however, we make our checks on an atomic vector instead. Maybe sth like span_r(mtcars, is.double) or similar would be better. (And add @AnoushiravanR to the list of contributors/acknowldgements for this finding.)