[SPARK-CORE][TEST][MINOR] Fix the wrong comment in test

## What changes were proposed in this pull request?

While learning core scheduler code, I found two lines of wrong comments. This PR simply corrects the comments.

## How was this patch tested?

Author: wm624@hotmail.com <wm624@hotmail.com>

Closes #15631 from wangmiao1981/Rbug.

core/src/test/scala/org/apache/spark/scheduler/TaskSetManagerSuite.scala

@@ -261,14 +261,14 @@ class TaskSetManagerSuite extends SparkFunSuite with LocalSparkContext with Logg
     assert(manager.resourceOffer("exec1", "host1", PROCESS_LOCAL) == None)
 
     clock.advance(LOCALITY_WAIT_MS)
-    // Offer host1, exec1 again, at NODE_LOCAL level: the node local (task 2) should
+    // Offer host1, exec1 again, at NODE_LOCAL level: the node local (task 3) should
     // get chosen before the noPref task
     assert(manager.resourceOffer("exec1", "host1", NODE_LOCAL).get.index == 2)
 
-    // Offer host2, exec3 again, at NODE_LOCAL level: we should choose task 2
+    // Offer host2, exec2, at NODE_LOCAL level: we should choose task 2
     assert(manager.resourceOffer("exec2", "host2", NODE_LOCAL).get.index == 1)
 
-    // Offer host2, exec3 again, at NODE_LOCAL level: we should get noPref task
+    // Offer host2, exec2 again, at NODE_LOCAL level: we should get noPref task
     // after failing to find a node_Local task
     assert(manager.resourceOffer("exec2", "host2", NODE_LOCAL) == None)
     clock.advance(LOCALITY_WAIT_MS)