Update accessibility.md to match updated Home Office accessibility standard

[edhamiltonHO]

will close #502

Content change

• Please review the accessibility checks for content changes.

I can confirm:

• Content does not include any code or configuration changes (excluding frontmatter information)
• Content meets the content standards
  e.g. Writing a principle and Writing a standard
• Content is suitable to open source, i.e.:
  • Content does not relate to unreleased gov policy
  • Content does not refer to anti-fraud mechanisms
  • Content does not include sensitive business logic
• Last updated date for content is correct