docs: fix import statements for plugin-nested-docs (#12494)

The module `@payloadcms/plugin-nested-docs/fields` does not seem to
exist (anymore). Instead `createParentField` and
`createBreadcrumbsField` are exported by
`@payloadcms/plugin-nested-docs`

Author: codeflorist

docs/plugins/nested-docs.mdx

@@ -180,8 +180,8 @@ and `createBreadcrumbField` methods. They will merge your customizations overtop
 
 ```ts
 import type { CollectionConfig } from 'payload'
-import { createParentField } from '@payloadcms/plugin-nested-docs/fields'
-import { createBreadcrumbsField } from '@payloadcms/plugin-nested-docs/fields'
+import { createParentField } from '@payloadcms/plugin-nested-docs'
+import { createBreadcrumbsField } from '@payloadcms/plugin-nested-docs'
 
 const examplePageConfig: CollectionConfig = {
   slug: 'pages',