Use valid example in Javadoc for @EnableWebFlux

This commit modifies the class-level Javadoc for the @EnableWebFlux
annotation to reference an actual method in WebFluxConfigurer.

Closes gh-23457

Author: Sauhardstark

spring-webflux/src/main/java/org/springframework/web/reactive/config/EnableWebFlux.java

@@ -47,10 +47,17 @@
  * @EnableWebFlux
  * @ComponentScan(basePackageClasses = MyConfiguration.class)
  * public class MyConfiguration implements WebFluxConfigurer {
+ * 
+ * 	   private ObjectMapper objectMapper;
  *
  * 	   @Override
- * 	   public void configureMessageWriters(List<HttpMessageWriter<?&gt&gt messageWriters) {
- *         messageWriters.add(new MyHttpMessageWriter());
+ * 	   public void configureHttpMessageCodecs(ServerCodecConfigurer configurer) {
+ *         configurer.defaultCodecs().jackson2JsonEncoder(
+ *             new Jackson2JsonEncoder(objectMapper)
+ *         );
+ *         configurer.defaultCodecs().jackson2JsonDecoder(
+ *             new Jackson2JsonDecoder(objectMapper)
+ *         );
  * 	   }
  *
  * 	   // ...