Fix build output dir for ref docs in CONTRIBUTING.md

Closes gh-26556

Author: cprayer

CONTRIBUTING.md

@@ -125,7 +125,7 @@ The reference documentation is in the [src/docs/asciidoc](src/docs/asciidoc) dir
 edit source files, and submit directly from GitHub.
 
 When making changes locally, execute `./gradlew asciidoctor` and then browse the result under
-`build/asciidoc/html5/index.html`.
+`build/docs/ref-docs/html5/index.html`.
 
 Asciidoctor also supports live editing. For more details read
 [Editing AsciiDoc with Live Preview](https://asciidoctor.org/docs/editing-asciidoc-with-live-preview/).