Merge branch 'rlehmann1-438371-update-syntax-hl-heading' into 'master'

Update syntax highlighting h3 in markdown.md

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/159614



Merged-by: Marcin Sedlak-Jakubowski <msedlakjakubowski@gitlab.com>
Approved-by: Marcin Sedlak-Jakubowski <msedlakjakubowski@gitlab.com>
Co-authored-by: Ryan Lehmann <rlehmann@gitlab.com>