What to do when you discover a mistake in the archived copy of your PhD thesis?
Though I am not in mathematics (I'm a philosopher), I would only add that generally speaking, the PhD dissertation should be one of the worst papers you ever publish. That doesn't mean it should be terrible...it only means that it's the start of your career, and your writing--and your research--will get better with experience.

You could still go ahead and submit a correction, but at the same time, don't stress yourself out over it. So many doctorandi today think the dissertation has to be flawless, and that they have to make some ground-breaking, Nobel-prize-worthy advancement in their field for the thesis to be any good. This is simply not true. Look--you have a postdoc, the results of which will no doubt be more influential on your future than your dissertation.

Again, I'm not saying to not submit the errata. Rather, I just don't want you to lose sleep over the mistake. Don't be too hard on yourself.