Fix RootRouteBuilder

[uhyo]

Discussed in #35

^{Originally posted by ka2n May 31, 2022}
Hello.

I would like to try to use RootRouteBuilder, but I could not get it to work.
Is there any mistake in my code?

https://codesandbox.io/s/eloquent-darkness-0ci2xs?file=%2Fsrc%2FApp.tsx