butterflies: use correct output filename for overlay layers

src/modules/butterflies/butterflies_actions.c

@@ -99,7 +99,7 @@ enum action_status bf_action_save_grid(struct cats_grid *grid, struct cats_confi
 enum action_status bf_action_save_overlay(struct cats_grid *grid, struct cats_configuration *conf)
 {
         int32_t id = grid->id;
-        char *filename = bf_population_filename(conf, grid);
+        char *filename = bf_population_overlay_filename(conf, grid);
         int module_id = CATS_MODULE_ID;
 
         struct grid_data_butterflies *module_data = grid->grid_modules[module_id].module_data;