[Minor] Fix a typo of type parameter in JavaUtils.scala

In JavaUtils.scala, thare is a typo of type parameter. In addition, the type information is removed at the time of compile by erasure.

This issue is really minor so I don't  file in JIRA.

Author: Kousuke Saruta <sarutak@oss.nttdata.co.jp>

Closes apache#3789 from sarutak/fix-typo-in-javautils and squashes the following commits:

e20193d [Kousuke Saruta] Fixed a typo of type parameter
82bc5d9 [Kousuke Saruta] Merge branch 'master' of git://git.apache.org/spark into fix-typo-in-javautils
99f6f63 [Kousuke Saruta] Fixed a typo of type parameter in JavaUtils.scala

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[A, _] =>
                   mm remove k
                   prev = None
                 case _ =>