fix: update Slack references to Discord

[anablaz]

Closes #accordproject/techdocs#388

This repository was listed among the repos that need an update of Slack references to Discord. I only updated the markdown file, not the historical meeting notes.

Changes

• Update Slack references to Discord in the markdown file

Related Issues

• Issue #Update Slack References to Discord techdocs#388

Author Checklist

• Ensure you provide a DCO sign-off for your commits using the --signoff option of git commit.
• Vital features and changes captured in unit and/or integration tests
• Commits messages follow AP format
• Extend the documentation, if necessary
• Merging to master from fork:branchname
• Manual accessibility test performed
  • Keyboard-only access, including forms
  • Contrast at least WCAG Level A
  • Appropriate labels, alt text, and instructions