fix: display the "contributing" section in the navbar

The file that was previously referenced in the Antora configuration
didn't exist. Use the right one.

Note: this is a port of #2748.

antora.yml

@@ -38,4 +38,4 @@ nav:
   - modules/runtime/runtime-nav.adoc
   - modules/security/security-nav.adoc
   - modules/version-update/version-update-nav.adoc
-  - modules/ROOT/build-community-nav.adoc
+  - modules/contributing/contributing-nav.adoc