Your driving instructor is right. Waiting for someone who is turning into an entirely different lane to complete their turn would be a bit too cautious. In fact, you completely followed the law. Here is, verbatim, what section 141 (6) of the Highway Traffic Act says:
Left turn, at intersection
(6) Where a driver or operator of a vehicle intends to turn to the left into an intersecting highway, he or she shall, where the highway on which he or she is driving has marked lanes for traffic, approach the intersection within the left-hand lane provided for the use of traffic moving in the direction in which his or her vehicle is proceeding or, where it has no such marked lanes, by keeping immediately to the right of the centre line of the highway and he or she shall make the left turn by entering the intersection to the right of the centre line or its extension and by leaving the intersection in the left-hand lane provided for the use of traffic moving in the direction in which his or her vehicle is proceeding where the lane is marked or, where no such lane is marked, by passing immediately to the right of the centre line of the intersecting highway.
He turned into the right lane, you turned into the left lane. You had right-of-way to the left lane. That's what you're supposed to do. If it all happened as you said, you should win in court without any real problems. Have you requested disclosure yet? This should get you the officer's notes.
Be prepared for the Crown to withdraw it at the last minute after a desperate attempt to get you to plea-bargain. If it does go to trial (not too likely), then question the officer, make sure that he admits that he stopped you for "not waiting," but get him to testify that you turned into the left lane and he turned into the right lane. Then read him section 141 (6), ask him where the "waiting" part is, and in conclusion say that the officer testified that you did exactly as section 141(6) and therefore you are not guilty.
The only scenario I could've seen is if you clearly were going to turn into the wrong lane, but it doesn't sound like it, from what you said.