Fix typo in help/error messages

[smelc]

Changelog

- description: |
    Fix typo in committee help and error messages
# uncomment types applicable to the change:
  type:
  # - feature        # introduces a new feature
  # - breaking       # the API has changed in a breaking way
  - compatible     # the API has changed but is non-breaking
  # - optimisation   # measurable performance improvements
  # - refactoring    # QoL changes
  # - bugfix         # fixes a defect
  # - test           # fixes/modifies tests
  # - maintenance    # not directly related to the code
  # - release        # related to a new release preparation
  # - documentation  # change in code docs, haddocks...

Context

Follow-up of @palas' remark: #816 (comment)

How to trust this PR

• Only changes help and error messages
• git grep Consitutional now returns 1

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• Self-reviewed the diff