Update class names in access control banner docs

[gosko]

The Access Control Banner component in the design system docs says to use access-control-banner--member for member-only pages, but that class does not exist in the CSS.

I believe those classes should be updated to match the classes used in the HTML example lower on the page: banner--access-control acl-member

[NicolaSaunders]

Proposed solution is at w3c/w3c-website-templates-bundle#154