[FLINK-21722][docs] Fix class reference in 'Generating Watermarks' pa…

…ge for Scala example

This closes apache#15142

docs/content/docs/dev/datastream/event-time/generating_watermarks.md

@@ -319,7 +319,7 @@ public class TimeLagWatermarkGenerator implements WatermarkGenerator<MyEvent> {
  * but only to a certain degree. The latest elements for a certain timestamp t will arrive
  * at most n milliseconds after the earliest elements for timestamp t.
  */
-class BoundedOutOfOrdernessGenerator extends AssignerWithPeriodicWatermarks[MyEvent] {
+class BoundedOutOfOrdernessGenerator extends WatermarkGenerator[MyEvent] {
 
  val maxOutOfOrderness = 3500L // 3.5 seconds
 
@@ -340,7 +340,7 @@ class BoundedOutOfOrdernessGenerator extends AssignerWithPeriodicWatermarks[MyEv
  * time by a fixed amount. It assumes that elements arrive in Flink after 
  * a bounded delay.
  */
-class TimeLagWatermarkGenerator extends AssignerWithPeriodicWatermarks[MyEvent] {
+class TimeLagWatermarkGenerator extends WatermarkGenerator[MyEvent] {
 
  val maxTimeLag = 5000L // 5 seconds
 
@@ -386,7 +386,7 @@ public class PunctuatedAssigner implements WatermarkGenerator<MyEvent> {
 {{< /tab >}}
 {{< tab "Scala" >}}
 ```scala
-class PunctuatedAssigner extends AssignerWithPunctuatedWatermarks[MyEvent] {
+class PunctuatedAssigner extends WatermarkGenerator[MyEvent] {
 
  override def onEvent(element: MyEvent, eventTimestamp: Long): Unit = {
  if (event.hasWatermarkMarker()) {