There isn't a way to rectify the mistake, at this point. It will be up to each application reader to assess what weight they will give this error. On the one hand, the mistake could signal a lack of sufficient proofreading, poor planning ahead (to give enough time for review), or both. On the other hand, in the grand scheme of the overall application, if everything else is strong, this kind of mistake is unlikely to have a major impact on your final decision.
I wouldn't worry about it, at this juncture.