docs: Clarify file location, add missing imports

docs/topics/snippets/customizing.md

@@ -44,7 +44,8 @@ class MemberFilterSet(WagtailFilterSet):
 And the following is the snippet's corresponding `SnippetViewSet` subclass:
 
 ```python
-from wagtail.admin.panels import FieldPanel
+# wagtail_hooks.py
+from wagtail.admin.panels import FieldPanel, ObjectList, TabbedInterface
 from wagtail.admin.ui.tables import UpdatedAtColumn
 from wagtail.snippets.models import register_snippet
 from wagtail.snippets.views.snippets import SnippetViewSet