Add parenthesis to side-effecting method

Per https://docs.scala-lang.org/scala3/book/methods-most.html#a-suggestion-about-methods-that-take-no-parameters

Author: barryoneill

src/main/g8/src/main/scala/Main.scala

@@ -1,4 +1,4 @@
-@main def hello: Unit =
+@main def hello(): Unit =
   println("Hello world!")
   println(msg)