Use standard name for main thread

DevTools look for a main thread named "main". The maven plugin is
actually using a different value and there is no real reason to. This
change fixes DevTools support as long as forking is enabled.

See gh-3315

spring-boot-tools/spring-boot-maven-plugin/src/main/java/org/springframework/boot/maven/RunMojo.java

@@ -53,7 +53,7 @@ public class RunMojo extends AbstractRunMojo {
 			throws MojoExecutionException {
 		IsolatedThreadGroup threadGroup = new IsolatedThreadGroup(startClassName);
 		Thread launchThread = new Thread(threadGroup, new LaunchRunner(startClassName,
-				arguments), startClassName + ".main()");
+				arguments), "main");
 		launchThread.setContextClassLoader(new URLClassLoader(getClassPathUrls()));
 		launchThread.start();
 		join(threadGroup);