GH-297 - Update to antora-ui-spring v0.3.7.

Author: rwinch

src/docs/antora/antora-playbook.yml

@@ -36,4 +36,4 @@ runtime:
     format: pretty
 ui:
   bundle:
-    url: https://github.com/spring-io/antora-ui-spring/releases/download/v0.3.5/ui-bundle.zip
+    url: https://github.com/spring-io/antora-ui-spring/releases/download/v0.3.7/ui-bundle.zip