Change PDF rename logic

Rework the rename logic used to include the reference PDF to try
and workaround the following local build error:

	Encountered duplicate path "reference/pdf/spring-boot-reference.pdf"
	during copy operation configured with DuplicatesStrategy.FAIL

See gh-20829

spring-boot-project/spring-boot-docs/build.gradle

@@ -240,7 +240,8 @@ task zip(type: Zip) {
 	}
 	from(asciidoctorPdf.outputDir) {
 		into "reference/pdf"
-		rename "index.pdf", "spring-boot-reference.pdf"
+		include "index.pdf"
+		rename { "spring-boot-reference.pdf" }
 	}
 	from(asciidoctorMultipage.outputDir) {
 		into "reference/html"