Fix broken asciidoc rendering

Restore the blank line at the end of `executable-jar-format.adoc`
which appears to be required.

See gh-18007

spring-boot-project/spring-boot-docs/src/main/asciidoc/appendix/executable-jar-format.adoc

@@ -293,3 +293,4 @@ If the preceding restrictions mean that you cannot use Spring Boot Loader, consi
 * http://www.jdotsoft.com/JarClassLoader.php[JarClassLoader]
 * https://sourceforge.net/projects/one-jar/[OneJar]
 * https://imperceptiblethoughts.com/shadow/[Gradle Shadow Plugin]
+