Harmonize property name

The property default format is lower case using hyphen. The JMX default
domain property has been harmonized to that format.

Fixes gh-2427

spring-boot-actuator/src/main/java/org/springframework/boot/actuate/autoconfigure/EndpointMBeanExportProperties.java

@@ -30,7 +30,7 @@ import org.springframework.util.StringUtils;
 @ConfigurationProperties(prefix = "endpoints.jmx")
 public class EndpointMBeanExportProperties {
 
-	@Value("${spring.jmx.default_domain:}")
+	@Value("${spring.jmx.default-domain:}")
 	private String domain;
 
 	private boolean uniqueNames = false;