Update custom-fields.md (#2028)

Replace outdated `.js` codebase references to `.ts`.

docusaurus/docs/dev-docs/custom-fields.md

@@ -371,5 +371,5 @@ export default {
 <!-- TODO: replace these tip and links by proper documentation of all the possible shapes and parameters for `options` -->
 
 :::tip
-The Strapi codebase gives an example of how settings objects can be described: check the [`baseForm.js`](https://github.com/strapi/strapi/blob/main/packages/core/content-type-builder/admin/src/components/FormModal/attributes/baseForm.js) file for the `base` settings and the [`advancedForm.js`](https://github.com/strapi/strapi/blob/main/packages/core/content-type-builder/admin/src/components/FormModal/attributes/advancedForm.js) file for the `advanced` settings. The base form lists the settings items inline but the advanced form gets the items from an [`attributeOptions.js`](https://github.com/strapi/strapi/blob/main/packages/core/content-type-builder/admin/src/components/FormModal/attributes/attributeOptions.js) file.
+The Strapi codebase gives an example of how settings objects can be described: check the [`baseForm.ts`](https://github.com/strapi/strapi/blob/main/packages/core/content-type-builder/admin/src/components/FormModal/attributes/baseForm.ts) file for the `base` settings and the [`advancedForm.ts`](https://github.com/strapi/strapi/blob/main/packages/core/content-type-builder/admin/src/components/FormModal/attributes/advancedForm.ts) file for the `advanced` settings. The base form lists the settings items inline but the advanced form gets the items from an [`attributeOptions.ts`](https://github.com/strapi/strapi/blob/main/packages/core/content-type-builder/admin/src/components/FormModal/attributes/attributeOptions.ts) file.
 :::