Simplify the hello-world exercise's stub. The goal of this is to ma…

…ke things easier for students to get started and to introduce as little syntax as possible. (#688)

exercises/practice/hello-world/src/main/scala/HelloWorld.scala

@@ -1,4 +1,4 @@
 object HelloWorld {
-  def hello() = ???
+  def hello() = "Goodbye, Mars!"
 }