CupertinoPicker minor documentation update

[LongCatIsLooong]

Description

Be more explicit about setting the initial item in CupertinoPicker's documentation.

Related Issues

Closes #43660

Tests

N/A

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[fluttergithubbot]

It looks like this pull request may not have tests. Please make sure to add tests before merging. If you need an exemption to this rule, contact Hixie.

Reviewers: Read the Tree Hygiene page and make sure this patch meets those guidelines before LGTMing.

[HansMuller]

LGTM

[justinmc]

LGTM 👍