Fix a few typos in scala.scalajs.js package object docs

Whether it's a mistake or not depends on the precise usage. In this case a hyphen is required, see rule #1 at https://owl.english.purdue.edu/owl/resource/576/01/ . They also explain that no hyphen is used "when compound modifiers come after a noun", e.g., "this language is dynamically typed." Apart from people making mistakes, that's probably another reason why you've seen it a lot without hyphen.