"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.
^Triage: return to pool. Assignee's commit bit was returned for safekeeping several years ago. Bugmeister note: to anyone working on this PR, it's probably fair to reset the maintainer (last action 8 years ago).
I will close this. igor stopped development time ago. Any change is unlikely to happen. Please feel free to reopen if you think otherwise.