Fix broken link to full reference to the Concerto metamodel in Metamodel JSON page

[anablaz]

Closes #35 and #34

Changes

• Update the link at the bottom of this page: https://concerto.accordproject.org/docs/design/specification/model-metamodel/

Related Issues

• Issue #35
• Issue #34

Author Checklist

• [X] 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
• [X] 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

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