Task "docsZip" copies duplicate reference files

Specify task "docsZip" source directory.
Issue: SPR-16789

gradle/docs.gradle

@@ -123,7 +123,7 @@ task docsZip(type: Zip, dependsOn: ['api', 'asciidoctor', 'dokka']) {
 		into "javadoc-api"
 	}
 
-	from (asciidoctor) {
+	from (asciidoctor.outputDir) {
 		into "spring-framework-reference"
 	}