silence a warning in the build definition

[SethTisue]

[warn] /Users/tisue/dotty/project/ScaladocGeneration.scala:16:20: non-variable type argument String in type pattern List[String] (the underlying of List[String]) is unchecked since it is eliminated by erasure
[warn]         case list: List[String] => s"$key:${list.map(escape).mkString(",")}"
[warn]                    ^

[som-snytt]

Could also just call .toString on elements.