Fix "Mapping Types" code snippet

[justinbaur]

The return type of mapping types code snippet does not match method signature and JsonSerializer parameter type misspelled.

@Bean
public ProducererFactory<Foo, Bar> kafkaProducerFactory(KafkaProperties properties,
    JsonSserializer customSerializer) {

    return new DefaultKafkaConsumerFactory<>(properties.buildProducerProperties(),
        customSerializer, customSerializer);
}

[artembilan]

Contribution is welcome!

Please, open the Pull Request with an appropriate change and we will continue discussion over there.

Thank you for heads up!

[justinbaur]

I have the issue updated in my fork was just adding for transparency and linking to PR. Thanks!