Fixed a typo of type parameter in JavaUtils.scala

Author: sarutak

core/src/main/scala/org/apache/spark/api/java/JavaUtils.scala

@@ -80,7 +80,7 @@ private[spark] object JavaUtils {
           prev match {
             case Some(k) =>
               underlying match {
-                case mm: mutable.Map[a, _] =>
+                case mm: mutable.Map[_, _] =>
                   mm remove k
                   prev = None
                 case _ =>