[SPARK-29348][SQL][FOLLOWUP] Fix slight bug on streaming example for Dataset.observe

Author: HeartSaVioR

sql/core/src/main/scala/org/apache/spark/sql/Dataset.scala

@@ -1891,14 +1891,10 @@ class Dataset[T] private[sql](
   * [[org.apache.spark.sql.util.QueryExecutionListener]] to the spark session.
   *
   * {{{
-  *   // Observe row count (rc) and error row count (erc) in the streaming Dataset
-  *   val observed_ds = ds.observe("my_event", count(lit(1)).as("rc"), count($"error").as("erc"))
-  *   observed_ds.writeStream.format("...").start()
-  *
   *   // Monitor the metrics using a listener.
   *   spark.streams.addListener(new StreamingQueryListener() {
   *     override def onQueryProgress(event: QueryProgressEvent): Unit = {
-  *       event.progress.observedMetrics.get("my_event").foreach { row =>
+  *       event.progress.observedMetrics.asScala.get("my_event").foreach { row =>
   *         // Trigger if the number of errors exceeds 5 percent
   *         val num_rows = row.getAs[Long]("rc")
   *         val num_error_rows = row.getAs[Long]("erc")
@@ -1908,7 +1904,12 @@ class Dataset[T] private[sql](
   *         }
   *       }
   *     }
+  *     def onQueryStarted(event: QueryStartedEvent): Unit = {}
+  *     def onQueryTerminated(event: QueryTerminatedEvent): Unit = {}
   *   })
+  *   // Observe row count (rc) and error row count (erc) in the streaming Dataset
+  *   val observed_ds = ds.observe("my_event", count(lit(1)).as("rc"), count($"error").as("erc"))
+  *   observed_ds.writeStream.format("...").start()
   * }}}
   *
   * @group typedrel