I saw somewhere that the grammatical structure of the official Slovenian translation of the title suggests that “Jedi” is indeed singular in this case (not speaking Slovenian, I can’t really weigh in myself). Anyway, I think it just refers to Luke at this particular point on the timeline. I don’t think that it has to preclude the possibility of more Jedi later on; Luke is simply the last Jedi as of his meeting with Rey in 35 ABY (or whatever it is). Anything can happen when the story progresses.