Polish Javadoc

WebSocketMessageBrokerStats's Javadoc now states the correct default
logging period.

Closes gh-1691

spring-websocket/src/main/java/org/springframework/web/socket/config/WebSocketMessageBrokerStats.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2017 the original author or authors.
+ * Copyright 2002-2018 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -39,7 +39,7 @@ import org.springframework.web.socket.messaging.SubProtocolWebSocketHandler;
  * {@code @EnableWebSocketMessageBroker} for Java config and
  * {@code <websocket:message-broker>} for XML.
  *
- * <p>By default aggregated information is logged every 15 minutes at INFO level.
+ * <p>By default aggregated information is logged every 30 minutes at INFO level.
  * The frequency of logging can be changed via {@link #setLoggingPeriod(long)}.
  *
  * <p>This class is declared as a Spring bean by the above configuration with the