Fix two comments since Actor is not used anymore.

## What changes were proposed in this pull request?

Fix two comments since Actor is not used anymore.

Author: Ding Fei <danis@danix>

Closes #15251 from danix800/comment-fixing.

core/src/main/scala/org/apache/spark/deploy/worker/WorkerWatcher.scala

@@ -21,7 +21,8 @@ import org.apache.spark.internal.Logging
 import org.apache.spark.rpc._
 
 /**
- * Actor which connects to a worker process and terminates the JVM if the connection is severed.
+ * Endpoint which connects to a worker process and terminates the JVM if the
+ * connection is severed.
  * Provides fate sharing between a worker and its associated child processes.
  */
 private[spark] class WorkerWatcher(

core/src/test/scala/org/apache/spark/MapOutputTrackerSuite.scala

@@ -253,7 +253,7 @@ class MapOutputTrackerSuite extends SparkFunSuite {
       rpcEnv.stop(masterTracker.trackerEndpoint)
       rpcEnv.setupEndpoint(MapOutputTracker.ENDPOINT_NAME, masterEndpoint)
 
-      // Frame size should be ~1.1MB, and MapOutputTrackerMasterActor should throw exception.
+      // Frame size should be ~1.1MB, and MapOutputTrackerMasterEndpoint should throw exception.
       // Note that the size is hand-selected here because map output statuses are compressed before
       // being sent.
       masterTracker.registerShuffle(20, 100)