Use canonical version of spellcheck

[gregcorbett]

• https://github.com/rojopolis/spellcheck-github-actions/blob/master/README.md#using-a-canonical-version
• so there is less noise/PRs to keep the action up to date, i.e.
  • Bump rojopolis/spellcheck-github-actions from 0.42.0 to 0.43.0 #33
  • Bump rojopolis/spellcheck-github-actions from 0.42.0 to 0.43.1 #34
  • Bump rojopolis/spellcheck-github-actions from 0.42.0 to 0.44.0 #35
  • Bump rojopolis/spellcheck-github-actions from 0.42.0 to 0.45.0 #36

[gregcorbett]

For reference, "Check for broken links" did initially fail, complaining about https://www.iris.ac.uk/ - even on a retry. Third time lucky (and enabling debug logging) and the check passed.