Dojo: you can bind %help but not unbind it

1. Run `=eny 'hello'` in Dojo, get error message "eny is immutable"
2. Run `=help 'hello'` in Dojo, this succeeds (but shouldn't)
3. Run `=help` to unbind, get error message "help is immutable"

This seems to be because `%help` is caught [on Line 563 but not Line 573.](https://github.com/urbit/urbit/blob/4b9f64e5a4ee3e7003a5aee896b1ff668eba2ca4/pkg/arvo/app/dojo.hoon#L556)