debug: the argument should be in lowercase

Author: amin1377

vpr/src/base/read_options.cpp

@@ -2443,7 +2443,7 @@ argparse::ArgumentParser create_arg_parser(std::string prog_name, t_options& arg
 
     route_grp.add_argument<bool, ParseOnOff>(args.flat_routing, "--flat_routing")
         .help("Enable VPR's flat routing (routing the nets from the source primitive to the destination primitive)")
-        .default_value("OFF")
+        .default_value("off")
         .show_in(argparse::ShowIn::HELP_ONLY);
 
     route_grp.add_argument(args.has_choking_spot, "--has_choking_spot")