I think I'd always understood the "Jedi" in the title to refer to Luke, either specifically or as a representation of the whole Jedi order. Yoda tells Luke that he will be a Jedi only after he confronts Vader. Since he does confront him, he earns the title. As he tells the Emperor, "I am a Jedi, like my father before me." By the end of the movie, the Sith have been defeated and the Emperor has lost his hold on the galaxy, so, in a broader sense of the title, the entire Jedi order can return from near extinction. Luke is free to follow Yoda's instructions and "pass on what you have learned."
Last year, I realized that the "Jedi" in the title could also refer to Anakin. He had been a Jedi and turned to the dark side, but near the end of the movie, he returns to the light side and becomes a Jedi again.
