I think you handled it correctly. It's up to the person paying to check the bill to make sure they're being charged the correct amount.
If I were the hostess in this kind of situation I wouldn't look at the bill too closely because I wouldn't care if I ended up paying a bit too much, so to me going through the entire bill to make sure everything was right wouldn't be worth the trouble. I wouldn't really appreciate being reminded to do something I'd actually decided not to do (although I'd certainly pretend to be grateful and I wouldn't mind it from a close friend or family member). If the hostess was someone who cared about not paying too much she'd be checking everything on the bill herself anyway so the reminder about your cancelled drink would be unnecessary in that case too.
You definitely shouldn't say anything about it now, since there's nothing she can do about it.
Wait, I am confused. How would they know to check the bill if it was 'right'? How are they supposed to read the OP's mind for this erroneous charge?
They wouldn't need to read her mind, they'd just need to pay a bit of attention to their surroundings and make a mental note of things like cancelled drinks which might make the bill more likely to be inaccurate. The OP didn't cancel the drink telepathically.
Or if they really
wanted to make sure they didn't pay any more than they had to they'd need to go through the whole bill and confirm with everyone at the table that every item was received, because mistakes can happen even when nobody cancels anything.