message: docs: update custom field extensions writing guide
diff: @@ -96,7 +96,7 @@ export const MyCustomFieldExtension = plugin.provide(
  ```tsx
  // packages/app/src/scaffolder/MyCustomExtension/index.ts
  -export { MyCustomFieldExtension } from './extension';
  +export { MyCustomFieldExtension } from './extensions';
  ```
  Once all these files are in place, you then need to provide your custom
  @@ -119,6 +119,8 @@ Should look something like this instead:
  ```tsx
  import { MyCustomFieldExtension } from './scaffolder/MyCustomExtension';
  +import { ScaffolderFieldExtensions } from '@backstage/plugin-scaffolder';
  +
  const routes = (
  <FlatRoutes>
  ...
  