Fix syntax of html_meta under myst field.
#18273
stevepiercyacceptance.yml
on: push
Matrix: Core Basic
Matrix: Core Basic - Plone 5
Matrix: Core Blocks
Matrix: Core Blocks - Listing
Matrix: Core Content
Matrix: Core Control Panels
Matrix: Coresandbox
Matrix: Core Volto Slate
Matrix: Deployment Mode
Matrix: Project Generator
Matrix: Multilingual in Deployment Mode
Matrix: Multilingual
Matrix: Working Copy