Final call: Math symbols example in Visible label is part of accessible name

Hey ya'all

This PR adds a test case for using mathematical symbols to "Visible label
is part of accessible name". If you have any concerns, please respond with
"changes request" to the pull request before October 30th.

https://github.com/act-rules/act-rules.github.io/pull/567

Kind regards,

-- 
*Wilco Fiers*
Axe for Web product owner - Co-facilitator WCAG-ACT - Chair ACT-R