docs: update examples with ts types

docs/admin/customizing-css.mdx

@@ -13,7 +13,7 @@ You can add your own CSS by providing your base Payload config with a path to yo
 To do so, provide your base Payload config with a path to your own stylesheet. It can be either a CSS or SCSS file.
 
 **Example in payload.config.js:**
-```js
+```ts
 import { buildConfig } from 'payload/config';
 import path from 'path';
 
@@ -21,7 +21,7 @@ const config = buildConfig({
 	admin: {
 		css: path.resolve(__dirname, 'relative/path/to/stylesheet.scss'),
 	},
-})
+});
 ```
 
 ### Overriding built-in styles