Fix GenericPageBreadcrumbsMixin to correctly take the specified number of items

The old code was taking the N-th item from the view's generic breadcrumbs instead of taking the last N items as the comment suggests

wagtail/admin/views/pages/utils.py

@@ -66,6 +66,7 @@ class GenericPageBreadcrumbsMixin:
         # which in most cases is the final item that links to the current view.
         # However, this can be customised in the case of generic views that are
         # nested inside another generic view.
-        return self.breadcrumbs_items + [
-            super().get_breadcrumbs_items()[-self.breadcrumbs_items_to_take]
-        ]
+        return (
+            self.breadcrumbs_items
+            + super().get_breadcrumbs_items()[-self.breadcrumbs_items_to_take :]
+        )