Adding example of cancelled operator (#29749)

Co-authored-by: Arnout Engelen <github@bzzt.net>
Co-authored-by: Renato Cavalcanti <renato@cavalcanti.be>

akka-docs/src/test/scala/docs/stream/operators/sink/Cancelled.scala

@@ -0,0 +1,22 @@
+/*
+ * Copyright (C) 2020 Lightbend Inc. <https://www.lightbend.com>
+ */
+
+package docs.stream.operators.sink
+
+import akka.NotUsed
+import akka.actor.ActorSystem
+import akka.stream.scaladsl.{ Sink, Source }
+
+import scala.concurrent.ExecutionContextExecutor
+
+object Cancelled {
+  implicit val system: ActorSystem = ???
+  implicit val ec: ExecutionContextExecutor = system.dispatcher
+  def cancelledExample(): NotUsed = {
+    //#cancelled
+    val source = Source(1 to 5)
+    source.runWith(Sink.cancelled)
+    //#cancelled
+  }
+}