Don't use procedure syntax

Because this is deprecated

Author: komainu8

Diff for: _overviews/tutorials/scala-for-java-programmers.md

@@ -330,7 +330,7 @@ inherited from `Object`.
 We can call overridden `toString` method as below.
 
     object ComplexNumbers {
-      def main(args: Array[String]) {
+      def main(args: Array[String]): Unit = {
         val c = new Complex(1.2, 3.4)
         println("Overridden toString(): " + c.toString())
       }