Fix checkstyle violations in samples

spring-boot-samples/spring-boot-sample-devtools/src/main/java/sample/devtools/Message.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2015 the original author or authors.
+ * Copyright 2012-2018 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.
@@ -16,8 +16,14 @@
 
 package sample.devtools;
 
-public class Message {
+public final class Message {
 
+	/**
+	 * Sample message.
+	 */
 	public static String MESSAGE = "Message";
 
+	private Message() {
+	}
+
 }