Clarify that cp behaves differently with 3 or more args

[omsai]

Closes #1506

• Set the expectation in the challenge question that they learner is not expected to know how more than 2 arguments to cp should behave and that this exercise is descriptive and exploratory.
• In the challenge explanation, clarify that the last argument must be a destination directory for 3 or more arguments to remove any confusion that there is anything special about the number 3 in using the example of 3 arguments.

[github-actions]

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🔍 Inspect the changes: https://github.com/swcarpentry/shell-novice/compare/md-outputs..md-outputs-PR-1507

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 03-create.md | 12 +++++++-----
 md5sum.txt   |  2 +-
 2 files changed, 8 insertions(+), 6 deletions(-)

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⏱️ Updated at 2025-09-15 11:36:05 +0000

[bkmgit]

Thanks.