Hi Andie,
If it were me.. I'd thank my lucky stars i noticed it now.
Prepare an explanation for the error... i.e. a correction ... stick it on the facing page of your annotated thesis (the one you'll be taking to you viva)
So, you're ready.
If its mentioned you can produce said explanation, acknowledge mistake and provide correction.
If its not mentioned... don't mention it.
If you can amend in your final version, just for completeness then do so...
All the very best for your viva.
Please come back and let us know how it goes.
Regards, Chuff