Update config example type in doc

Examples show `KafkaTemplate<Integer, String>` and `ProducerFactory<Integer, String>`, but sample config had `ProducerConfig.KEY_SERIALIZER_CLASS_CONFIG` set as `StringSerializer.class`

spring-kafka-docs/src/main/asciidoc/kafka.adoc

@@ -238,7 +238,7 @@ public ProducerFactory<Integer, String> producerFactory() {
 public Map<String, Object> producerConfigs() {
     Map<String, Object> props = new HashMap<>();
     props.put(ProducerConfig.BOOTSTRAP_SERVERS_CONFIG, "localhost:9092");
-    props.put(ProducerConfig.KEY_SERIALIZER_CLASS_CONFIG, StringSerializer.class);
+    props.put(ProducerConfig.KEY_SERIALIZER_CLASS_CONFIG, IntegerSerializer.class);
     props.put(ProducerConfig.VALUE_SERIALIZER_CLASS_CONFIG, StringSerializer.class);
     // See https://kafka.apache.org/documentation/#producerconfigs for more properties
     return props;