Fix pkgdown articles reference for subdirectory vignette

Vignettes in vignettes/articles/ must be referenced with the
articles/ prefix in _pkgdown.yml contents.

Author: cafferychen777

R/_pkgdown.yml

@@ -244,7 +244,7 @@ articles:
     contents:
       - usage-tutorial
       - consensus-principles
-      - visualization-guide
+      - articles/visualization-guide
   - title: Advanced Topics
     navbar: Advanced Topics
     contents: