listitemmanager: Fix section change handler mistake

The statement is not doing what it was meant to do.

gtk_list_item_manager_get_nth (self, position, &offset) returns the
tile for a given position, and if the tile maps to more than 1 item,
the offset indicates how far into that tile the given position is.

So position - offset would give us the position of this tile. It
doesn't make sense to subtract it from n_items.

Instead, we should be adding the offset to compensate for having
landed too early in the list, such that we successfully reach
position + n_items.

gtk/gtklistitemmanager.c

@@ -1625,7 +1625,7 @@ gtk_list_item_manager_model_sections_changed_cb (GListModel         *model,
   gtk_list_item_change_clear_header (&change, &header->widget);
   gtk_list_tile_set_type (header, GTK_LIST_TILE_UNMATCHED_HEADER);
 
-  n_items -= MIN (n_items, position - offset);
+  n_items += offset;
   while (n_items > 0)
     {
       switch (tile->type)