Restore System property in Logging section of the reference documentation

Before this commit, it is rendered as an unexpected link with simple class
name, it should be full qualified class name.

See gh-43341

Author: quaff

spring-boot-project/spring-boot-docs/src/docs/antora/modules/reference/pages/features/logging.adoc

@@ -287,7 +287,7 @@ logging:
 
 The various logging systems can be activated by including the appropriate libraries on the classpath and can be further customized by providing a suitable configuration file in the root of the classpath or in a location specified by the following Spring javadoc:org.springframework.core.env.Environment[] property: configprop:logging.config[].
 
-You can force Spring Boot to use a particular logging system by using the javadoc:org.springframework.boot.logging.LoggingSystem[] system property.
+You can force Spring Boot to use a particular logging system by using the `org.springframework.boot.logging.LoggingSystem` system property.
 The value should be the fully qualified class name of a javadoc:org.springframework.boot.logging.LoggingSystem[] implementation.
 You can also disable Spring Boot's logging configuration entirely by using a value of `none`.