[SPARK-23888][CORE] correct the comment of hasAttemptOnHost()

TaskSetManager.hasAttemptOnHost had a misleading comment.  The comment
said that it only checked for running tasks, but really it checked for
any tasks that might have run in the past as well.  This updates to line
up with the implementation.

Author: wuyi <ngone_5451@163.com>

Closes #20998 from Ngone51/SPARK-23888.

core/src/main/scala/org/apache/spark/scheduler/TaskSetManager.scala

@@ -287,7 +287,7 @@ private[spark] class TaskSetManager(
     None
   }
 
-  /** Check whether a task is currently running an attempt on a given host */
+  /** Check whether a task once ran an attempt on a given host */
   private def hasAttemptOnHost(taskIndex: Int, host: String): Boolean = {
     taskAttempts(taskIndex).exists(_.host == host)
   }