doc Replace the word stage by operator in stream doc (#25138)

akka-docs/src/test/scala/docs/stream/GraphStageDocSpec.scala

@@ -287,14 +287,14 @@ class GraphStageDocSpec extends AkkaSpec {
   "Demonstrate chaining of graph stages" in {
     val sink = Sink.fold[List[Int], Int](List.empty[Int])((acc, n) ⇒ acc :+ n)
 
-    //#graph-stage-chain
+    //#graph-operator-chain
     val resultFuture = Source(1 to 5)
       .via(new Filter(_ % 2 == 0))
       .via(new Duplicator())
       .via(new Map(_ / 2))
       .runWith(sink)
 
-    //#graph-stage-chain
+    //#graph-operator-chain
 
     Await.result(resultFuture, 3.seconds) should ===(List(1, 1, 2, 2))
   }

akka-docs/src/test/scala/docs/stream/GraphStageLoggingDocSpec.scala

@@ -15,7 +15,7 @@ class GraphStageLoggingDocSpec extends AkkaSpec("akka.loglevel = DEBUG") {
   implicit val materializer = ActorMaterializer()
   implicit val ec = system.dispatcher
 
-  //#stage-with-logging
+  //#operator-with-logging
   import akka.stream.stage.{ GraphStage, GraphStageLogic, OutHandler, StageLogging }
 
   final class RandomLettersSource extends GraphStage[SourceShape[String]] {
@@ -39,7 +39,7 @@ class GraphStageLoggingDocSpec extends AkkaSpec("akka.loglevel = DEBUG") {
     def nextChar(): Char =
       ThreadLocalRandom.current().nextInt('a', 'z'.toInt + 1).toChar
   }
-  //#stage-with-logging
+  //#operator-with-logging
 
   "demonstrate logging in custom graphstage" in {
     val n = 10