demo route note in replay

selfdrive/ui/replay/main.cc

@@ -8,6 +8,7 @@ int main(int argc, char *argv[]){
   QString route(argv[1]);
   if (route == "") {
     printf("Usage: ./replay \"route\"\n");
+    printf("  For a public demo route, use '3533c53bb29502d1|2019-12-10--01-13-27'\n");
     return 1;
   }