Simplify the example used for case objects. (#2172)

Co-authored-by: Julien Richard-Foy <[email protected]>
Co-authored-by: Seth Tisue <[email protected]>

Author: 3 people

_overviews/scala3-book/domain-modeling-tools.md

@@ -685,14 +685,14 @@ case class DecreaseVolume(amount: Int) extends Message
 case object StopPlaying extends Message
 ```
 
-Then in other parts of your code you can write methods like this, which use pattern matching to handle the incoming message:
+Then in other parts of your code, you can write methods like this, which use pattern matching to handle the incoming message (assuming the methods `playSong`, `changeVolume`, and `stopPlayingSong` are defined somewhere else):
 
 ```scala
-def handleMessages(msg: Message) = message match
-  case PlaySong(name)      => playSong(name)
-  case IncreaseVolume(amt) => changeVolume(amt)
-  case DecreaseVolume(amt) => changeVolume(-amt)
-  case StopPlaying         => stopPlayingMusic
+def handleMessages(message: Message): Unit = message match
+  case PlaySong(name)         => playSong(name)
+  case IncreaseVolume(amount) => changeVolume(amount)
+  case DecreaseVolume(amount) => changeVolume(-amount)
+  case StopPlaying            => stopPlayingSong()
 ```
 [ref-enums]: {{ site.scala3ref }}/enums/enums.html
 [adts]: {% link _overviews/scala3-book/types-adts-gadts.md %}