I’ve seen this kind of thing before. I think some wiki or cms engines tend to output minus sign as the longer dash whatever it is called. Then some terminal emulators seem to recognize it as double minus when you paste it into them, iirc.
However it should be single minus ‘nice -10’
although setting the niceness shouldn’t have that big of an impact compared to rtprio anyway.
also neither renoise or jackd doesn’t seem to set the nice level themselves, so it shouldn’t really matter unless you’re going to renice the processes yourself.