"n'th" is a valid alternative for "nth": https://en.wiktionary.org/wiki/nth . I don't believe that igor should be flagging this as a typo.
"Th" is flagged as a spelling error because it is a misspelling found in FreeBSD documents. I would say the best way to write that is Nth, distinguishing the N from the rest of it. I'm hesitant to remove it from the list of misspelled words because a bare "th" is almost always a spelling error. A good argument could be made for removing both it and "fr" on the basis that they are country codes.
In this case, I would still prefer the "Nth" form.