Fix "Flux.create" README.md example (#3600)

The example used an incorrect `timeout()` method with a number of seconds as a parameter. A `Duration` object is now passed as the argument.

README.md

@@ -210,7 +210,7 @@ Flux.create(sink -> {
     },
     // Overflow (backpressure) handling, default is BUFFER
     FluxSink.OverflowStrategy.LATEST)
-    .timeout(3)
+    .timeout(Duration.ofSeconds(3))
     .doOnComplete(() -> System.out.println("completed!"))
     .subscribe(System.out::println)
 ```