Upgrade to Antora UI 0.4.18.

See gh-913

Author: mp911de

src/main/antora/antora-playbook.yml

@@ -32,5 +32,5 @@ runtime:
     format: pretty
 ui:
   bundle:
-    url: https://github.com/spring-io/antora-ui-spring/releases/download/v0.4.16/ui-bundle.zip
+    url: https://github.com/spring-io/antora-ui-spring/releases/download/v0.4.18/ui-bundle.zip
     snapshot: true