PhD viva reference list mistake
Don't worry, it's a very minor issue. In the worst case it will be part of some minor corrections you'll have to do after the viva.

It's important that you don't contact the examiners yourself, as this might be against the rules. Instead talk to your supervisor, they will know what can be done in compliance with the institution rules; and even if they don't they will know who to ask at least. 

As far as I know it's usually fine for the supervisor to contact the examiners, but rules may differ by institution, and since the PhD viva is an official examination it's crucial to do things by the book.