If the HSA administrator accepts your request to handle a mistaken distribution (they don't have to), then they are required to issue a corrected 1099-SA. You can use this corrected 1099-SA to amend your 2018 return.
Of course, when you do the mistaken distribution application with the HSA administrator (look to see if the application is on their website), you will have to send them a check for that amount. But going through this process is important so that the HSA administrator doesn't list this check as a contribution but as a repayment of a mistaken distribution.
