ICU-21396 fix typo in example

[alurie]

Checklist

• [x] Issue filed: https://unicode-org.atlassian.net/browse/ICU-21396
• [x] Updated PR title and link in previous line to include Issue number
• [ ] Issue accepted
• [ ] Tests included
• [x] Documentation is changed or added

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[markusicu]

Thanks for the PR -- assigned to Mark who best knows the transform syntax.

Please remember to (a) sign the CLA and (b) amend your commit with ICU-21396