Abstract methods and override issues

[christydennison]

object-oriented-programming/abstract-and-sealed-classes#abstract-methods-and-override

1. You don't explain override, but you use it as if it were a keyword in the title. There is an exercise called "overriding" but that doesn't imply that we should use the keyword.
2. You're defining classes within the object Message, but don't really explain why you would do that instead of just defining them outside of the object. It doesn't seem to affect anything when you define the case classes outside of the object.