GH-49107: [C++] Doc update in aggregate_basic for first-last

[FrenchCommando]

fix consistency issues in doc between the first - last - first/last functions

Rationale for this change

small change in docs
there seems to be some copy-paste residuals from the 3 version of the function

What changes are included in this PR?

Doc change

Are these changes tested?

No

Are there any user-facing changes?

Just doc

• GitHub Issue: [C++] Doc update in aggregate_basic for first-last #49107

[github-actions]

Thanks for opening a pull request!

If this is not a minor PR. Could you open an issue for this pull request on GitHub? https://github.com/apache/arrow/issues/new/choose

Opening GitHub issues ahead of time contributes to the Openness of the Apache Arrow project.

Then could you also rename the pull request title in the following format?

GH-${GITHUB_ISSUE_ID}: [${COMPONENT}] ${SUMMARY}

or

MINOR: [${COMPONENT}] ${SUMMARY}

See also:

• Other pull requests
• Contribution Guidelines - Contributing Overview

[github-actions]

⚠️ GitHub issue #49107 has been automatically assigned in GitHub to PR creator.