Polish "Add reference to Mule 4 starter"

See gh-20915

Author: snicoll

spring-boot-project/spring-boot-starters/README.adoc

@@ -133,7 +133,7 @@ do as they were designed before this was clarified.
 | https://logback.qos.ch/access.html[Logback-access]
 | https://github.com/akihyro/logback-access-spring-boot-starter
 
-| Mule 4
+| https://github.com/mulesoft/mule[Mule 4]
 | https://github.com/hawkore/mule4-spring-boot-starter
 
 | https://github.com/mybatis/mybatis-3[MyBatis]