create, README: fix descriptions of new options

[sanderploegsma]

The help text for the --practice-exercise and --concept-exercise options of configlet create were inverted:

Options for create:
      --approach <slug>           The slug of the approach
      --article <slug>            The slug of the article
      --practice-exercise <slug>  The slug of the concept exercise
      --concept-exercise <slug>   The slug of the practice exercise
  -e, --exercise <slug>           Only operate on this exercise
  -o, --offline                   Do not update the cached 'problem-specifications' data

This PR swaps them around.

[sanderploegsma]

It looks like the CI failure is unrelated to the changes, not sure if it fails randomly more often. I can't seem to trigger a re-run myself though.

[ee7]

Thanks!

It looks like the CI failure is unrelated to the changes

Yup, sorry. The Windows build is flaky, and we haven't yet diagnosed the cause: #829