Update packages/kit/src/core/config/options.js

Co-authored-by: Ben McCann <322311+benmccann@users.noreply.github.com>

Author: Rich-Harris

packages/kit/src/core/config/options.js

@@ -187,7 +187,7 @@ const options = object(
 				concurrency: number(1),
 				crawl: boolean(true),
 				createIndexFiles: error(
-					(keypath) => `${keypath} has been removed — configure it via your adapter instead`
+					(keypath) => `${keypath} has been removed — it is now controlled by the trailingSlash option. See https://kit.svelte.dev/docs/configuration#trailingslash`
 				),
 				enabled: boolean(true),
 				entries: validate(['*'], (input, keypath) => {