It's ambiguous who the titular Jedi in Return of the Jedi is. Originally, I thought it referred 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, and the order is revived in him. 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."
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.