ICU-21834 Bad link to build in ICU FAQs

[dehilsterlexis]

Signed-off-by: David de Hilster [email protected]

Checklist

• [x] Required: Issue filed: https://unicode-org.atlassian.net/browse/ICU-21834
• [x] Required: The PR title must be prefixed with a JIRA Issue number.
• [x] Required: The PR description must include the link to the Jira Issue, for example by completing the URL in the first checklist item
• [x] Required: Each commit message must be prefixed with a JIRA Issue number.
• [x] Issue accepted (done by Technical Committee after discussion)
• [ ] Tests included, if applicable
• [x] API docs and/or User Guide docs changed or added, if applicable

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[dehilsterlexis]

@echeran for review