Fix link to spring-boot-maven-plugin

README.md

 $ mvn package
 ```
 
-[spring-boot-maven-plugin]: https://github.com/SpringSource/spring-boot/tree/master/spring-boot-maven-plugin
+[spring-boot-maven-plugin]: https://github.com/SpringSource/spring-boot/tree/master/spring-boot-tools/spring-boot-maven-plugin
 
 > **Note:** The procedure above will create a runnable JAR. You can also opt to [build a classic WAR file](/guides/gs/convert-jar-to-war/) instead.