You can contact immigration or call them to ask them to cancel the visa their end so that you can redo the ETA and repay again. They will not change the details for you (your mistake) unless you get someone who is not a "jobsworth". Alternately, you could just take the ETA you have with you, see if the immigration officer picks up on the mistake, if they do just play dumb he might direct you to an officer to change the details or buy a new visa there and then within the airport for the visa fee.