Fix DEBUG error message about reply topics

**cherry-pick to 2.1.x**

Author: garyrussell

spring-kafka/src/main/java/org/springframework/kafka/requestreply/ReplyingKafkaTemplate.java

@@ -114,7 +114,8 @@ else if (properties.getTopicPartitions() != null && properties.getTopicPartition
 			this.replyTopic = null;
 			this.replyPartition = null;
 			this.logger.debug("Could not determine container's reply topic/partition; senders must populate "
-					+ "at least the " + KafkaHeaders.REPLY_PARTITION + " header");
+					+ "at least the " + KafkaHeaders.REPLY_TOPIC + " header, and optionally the "
+					+ KafkaHeaders.REPLY_PARTITION + " header");
 		}
 		else {
 			this.replyTopic = replyTopic.getBytes(StandardCharsets.UTF_8);