chore (docs) remove —ergo flag from concerto validate command docs (#610)

[jamieshorten]

Remove the --ergo flag from the concerto-validate documentation now that it is being removed in Concerto

Related Issues

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Author Checklist

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

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