Fix typo in logback example

See gh-18295

Author: brabenetz

spring-boot-project/spring-boot-docs/src/main/asciidoc/howto.adoc

@@ -1308,7 +1308,7 @@ A typical custom `logback.xml` file would look something like this:
 ----
 	<?xml version="1.0" encoding="UTF-8"?>
 	<configuration>
-		<include resource="org/springframework/boot/logging/logback/default.xml"/>
+		<include resource="org/springframework/boot/logging/logback/defaults.xml"/>
 		<include resource="org/springframework/boot/logging/logback/console-appender.xml" />
 		<root level="INFO">
 			<appender-ref ref="CONSOLE" />