show_narration: use label column if only given a string

This matches behaviour in 1.14

src/scripting/lua_gui2.cpp

@@ -86,7 +86,7 @@ int show_message_dialog(lua_State* L)
 			t_string short_opt;
 			config opt;
 			if(luaW_totstring(L, -1, short_opt)) {
-				opt["description"] = short_opt;
+				opt["label"] = short_opt;
 			} else if(!luaW_toconfig(L, -1, opt)) {
 				std::ostringstream error;
 				error << "expected array of config and/or translatable strings, but index ";