Fix Gradle Shadow Plugin link in reference documentation

See gh-45720

Signed-off-by: Tran Ngoc Nhan <[email protected]>

Author: ngocnhan-tran1996

spring-boot-project/spring-boot-docs/src/docs/antora/modules/specification/pages/executable-jar/alternatives.adoc

@@ -6,5 +6,5 @@ If the preceding restrictions mean that you cannot use Spring Boot Loader, consi
 * https://maven.apache.org/plugins/maven-shade-plugin/[Maven Shade Plugin]
 * http://www.jdotsoft.com/JarClassLoader.php[JarClassLoader]
 * https://sourceforge.net/projects/one-jar/[OneJar]
-* https://imperceptiblethoughts.com/shadow/[Gradle Shadow Plugin]
+* https://gradleup.com/shadow/[Gradle Shadow Plugin]