Fix countString

Author: odersky

compiler/src/dotty/tools/dotc/reporting/Reporter.scala

@@ -314,7 +314,7 @@ abstract class Reporter extends interfaces.ReporterResult {
   /** Returns a string meaning "n elements". */
   protected def countString(n: Int, elements: String): String = n match {
     case 0 => s"no ${elements}s"
-    case 1 => s"one ${elements}s"
+    case 1 => s"one ${elements}"
     case 2 => s"two ${elements}s"
     case 3 => s"three ${elements}s"
     case 4 => s"four ${elements}s"