Minor command line bug fixes for flow-over-heated-plate/solid-mfem

[j-signorelli]

Very minor fix of command line argument <--> variable associations.

Checklist

• I added a summary of any user-facing changes (compared to the last release) in the changelog-entries/<PRnumber>.md.
• If I changed requirements.txt files, I regenerated sibling requirements-reference.txt files with python3 .dev-tools/releasing/update-requirements-reference.py (pass a path to update only that directory, or --all to refresh everything).

For new tutorials or tutorial cases:

• I followed the tutorial folder structure
• I added/updated the tutorial README.md
• I added/updated the tutorial metadata.yaml
• I added tests in tests/tests.yaml
• I submitted a pull request to the website with:
  • An entry in _config.yaml
  • A sidebar entry
  • An entry in the tutorials overview

[MakisH]

Thanks a lot!
I checked the run.sh, and these are not used in the tutorial, so I assume this will not affect the results. If yes, we will find out in the nightly tests.