pluralize possible values in `print.qual_param`

[simonpcouch]

"2 possible value include" in output of:

dials::new_qual_param(
  type = "character",
  values = c("rmse", "logloss"),
  label = c(eval_metric = "Evaluation Metric")
)
#> Evaluation Metric  (qualitative)
#> 2 possible value include:
#> 'rmse' and 'logloss'

^{Created on 2022-08-15 by the reprex package (v2.0.1)}