use slash in URL

references scala/scala-lang#1732

Author: SethTisue

src/main/scala/MakeReleaseNotes.scala

@@ -110,7 +110,7 @@ object MakeReleaseNotes {
     </html>"""
       case MarkDown => s"""---
 category: announcement
-permalink: /news/$version
+permalink: /news/$version/
 title: "Scala ${currentTag drop 1} is now available!"
 ---
 ${rawHandWrittenNotes()}