improve spacing in pretty methods

[EmilHvitfeldt]

A number of pretty methods returns strings with trailing spaces.

Below, it would be better for line 58 to not have a trailing space, and for 61 to start with a leading space

https://github.com/tidymodels/rsample/blob/d393a8a08d653071fa27e2b67260a27da191bb59/R/printing.R#L49-L64

And very few of these methods are tested