MINOR: Add a newline to a Javadoc sample in KafkaConsumer

This PR simply adds a newline to a Javadoc sample in `KafkaConsumer` to flush the outputs.

Author: Johnny Lim <izeye@naver.com>

Reviewers: Ewen Cheslack-Postava <ewen@confluent.io>

Closes #1390 from izeye/patch-1

clients/src/main/java/org/apache/kafka/clients/consumer/KafkaConsumer.java

@@ -195,7 +195,7 @@ import java.util.regex.Pattern;
  *     while (true) {
  *         ConsumerRecords<String, String> records = consumer.poll(100);
  *         for (ConsumerRecord<String, String> record : records)
- *             System.out.printf("offset = %d, key = %s, value = %s", record.offset(), record.key(), record.value());
+ *             System.out.printf("offset = %d, key = %s, value = %s%n", record.offset(), record.key(), record.value());
  *     }
  * </pre>
  *