they have info on the page http://admission.universityofcalifornia.edu/how-to-apply/after-you-apply/index.html that seems to say you can update info after you apply. And it also gives an email address to contact them.
As a transfer you will be doing an update in a few weeks to update with your fall grades and any class changes so you can correct it then. However if it was me I’d email admissions at the schools where I applied and mention a class was accidentally omitted and then you’ll fix it in the update a few weeks down the road. Sending these emails isn’t going to hurt anything but it may ward off some potential issues (for example suppose your app with the missing class, and remember that’s all the non-TAG schools will get, does not have enough units to make you eligible to transfer).