Publish distribution zip files again

A change in `docs.gradle` in 5.2.4 resulted in distribution zip files
no longer being published to
https://repo.spring.io/release/org/springframework/spring/.

This commit fixes this by updating the `mavenContent` configuration as
suggested by @wilkinsona.

Closes gh-24605

gradle/docs.gradle

@@ -10,7 +10,7 @@ repositories {
 	maven {
 		url "https://repo.spring.io/release"
 		mavenContent {
-			group "io.spring.asciidoctor"
+			includeGroup "io.spring.asciidoctor"
 		}
 	}
 }