Call for review: Add cross-table headers attribute example

Hey folks,
This pull request adds an example showing how the headers attribute can not
be used to link between two table elements. This addition was requested by
the Accessibility Guidelines Working Group before the rule is approved for
publication. If you have any comments, please leave a review before June
23rd:

https://github.com/act-rules/act-rules.github.io/pull/1856/files

Kind regards

-- 
*Wilco Fiers*
Axe-core & Axe-linter product owner - WCAG 3 Project Manager - Facilitator
ACT Task Force