Fix description of checks for "bypass" Lighthouse audit based on Axe

[thibaudcolas]

This audit is based on Axe’s bypass rule, which checks headings, not header elements. The error might stem from Axe’s check for this being called header-present, but it does check h1…h6 elements, not <header>.

I looked for similar copy elsewhere and this seems to be the only place with the issue.