Fix the setup() function in the documentation (#22368)

#16018

Author: leafac

docs/docs/adding-an-rss-feed.md

@@ -159,7 +159,8 @@ module.exports = {
           }
         `,
         /* highlight-start */
-        setup: () => ({
+        setup: options => ({
+          ...options,
           custom_namespaces: {
             itunes: 'http://www.itunes.com/dtds/podcast-1.0.dtd',
           },