Gh-2921: Fix bean registration in docs sample

Fixes #2921

The `RoutingKafkaTemplate` sample configuration in the `sending-messages.adoc`
use an out-dated API for `context.registerBean()`
(Or the sample has never been correct)

spring-kafka-docs/src/main/antora/modules/ROOT/pages/kafka/sending-messages.adoc

@@ -247,7 +247,7 @@ public class Application {
         Map<String, Object> configs = new HashMap<>(pf.getConfigurationProperties());
         configs.put(ProducerConfig.VALUE_SERIALIZER_CLASS_CONFIG, ByteArraySerializer.class);
         DefaultKafkaProducerFactory<Object, Object> bytesPF = new DefaultKafkaProducerFactory<>(configs);
-        context.registerBean(DefaultKafkaProducerFactory.class, "bytesPF", bytesPF);
+        context.registerBean("bytesPF", DefaultKafkaProducerFactory.class, () -> bytesPF);
 
         Map<Pattern, ProducerFactory<Object, Object>> map = new LinkedHashMap<>();
         map.put(Pattern.compile("two"), bytesPF);