butterflies: fix typo

src/modules/butterflies/butterflies_main.c

@@ -98,7 +98,7 @@ void load_butterflies_species_params(struct cats_configuration *conf, struct cat
                 check_and_create_directory_if_needed(path);
                 struct string_array *name = new_string_array_init("rw-debug");
                 string_array_add(name, conf->run_name);
-                data->debug_rw_filename = assemble_filename(path, name, "_", ".csv");
+                data->debug_rw_filename = assemble_filename(path, name, "_", "csv");
                 data->debug_rw_file = fopen(data->debug_rw_filename, "w");
                 ENSURE_FILE_OPENED(data->debug_rw_file, data->debug_rw_filename);
                 free_string_array(&path);