Spreadsheet: undo accidental icon change

This was changed as part of 4e958617a9.
While we likely want to use a different icon in the future,
it still has to be choosen or designed.

source/blender/editors/space_spreadsheet/spreadsheet_data_source_geometry.cc

@@ -560,7 +560,7 @@ int get_instance_reference_icon(const bke::InstanceReference &reference)
       return ICON_OUTLINER_COLLECTION;
     }
     case bke::InstanceReference::Type::GeometrySet: {
-      return ICON_THREE_DOTS;
+      return ICON_EMPTY_AXIS;
     }
     case bke::InstanceReference::Type::None: {
       break;