[CORE][MINOR] Correct a log message

Author: madianjun

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

@@ -1521,8 +1521,8 @@ private[spark] class DAGScheduler(
             markStageAsFinished(failedStage, errorMessage = Some(failureMessage),
               willRetry = !shouldAbortStage)
           } else {
-            logDebug(s"Received fetch failure from $task, but its from $failedStage which is no " +
-              s"longer running")
+            logDebug(s"Received fetch failure from $task, but it's from $failedStage which is no " +
+              "longer running")
           }
 
           if (mapStage.rdd.isBarrier()) {