[web] fix button tertiary colors

Summary: I made some small errors with the button tertiary colors that got caught in D10549 and ENG-6353, where I created two identical color variables for --button-label-tertiary-default + --button-label-secondary-defaul and I misspelled tertiary for --button-background-tetirary-hover. This diff addresses both of those mistakes

Test Plan: flow + confirmed there were no regressions

Reviewers: atul, inka

Reviewed By: inka

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10562