This stuck out to me too, but some googling of "Illonois" brings up a lot of census records from 1930 using that spelling. I can't find any concrete confirmation but maybe it's intentional and accurate?

Wikipedia says "'Illinois' is the modern spelling for the early French missionaries and explorers' name for the Illinois people, a name that was spelled in many different ways in the early records." and references http://www.illinoisprairie.info/Eriniouaj.htm