[Docs] Fix imports in "Buttons with dropdown lists" hooks example

CONTRIBUTORS.rst

@@ -264,6 +264,7 @@ Contributors
 * Martin Sandström
 * Adrian Turjak
 * Michael Palmer
+* Philipp Bosch
 
 Translators
 ===========

docs/reference/hooks.rst

@@ -740,6 +740,8 @@ Buttons with dropdown lists
 
   .. code-block:: python
 
+    from wagtail.admin import widgets as wagtailadmin_widgets
+
     @hooks.register('register_page_listing_buttons')
     def page_custom_listing_buttons(page, page_perms, is_parent=False):
         yield wagtailadmin_widgets.ButtonWithDropdownFromHook(
@@ -754,11 +756,11 @@ Buttons with dropdown lists
     @hooks.register('my_button_dropdown_hook')
     def page_custom_listing_more_buttons(page, page_perms, is_parent=False):
         if page_perms.can_move():
-            yield Button('Move', reverse('wagtailadmin_pages:move', args=[page.id]), priority=10)
+            yield wagtailadmin_widgets.Button('Move', reverse('wagtailadmin_pages:move', args=[page.id]), priority=10)
         if page_perms.can_delete():
-            yield Button('Delete', reverse('wagtailadmin_pages:delete', args=[page.id]), priority=30)
+            yield wagtailadmin_widgets.Button('Delete', reverse('wagtailadmin_pages:delete', args=[page.id]), priority=30)
         if page_perms.can_unpublish():
-            yield Button('Unpublish', reverse('wagtailadmin_pages:unpublish', args=[page.id]), priority=40)
+            yield wagtailadmin_widgets.Button('Unpublish', reverse('wagtailadmin_pages:unpublish', args=[page.id]), priority=40)