ICU-21396 Fix incorrect result shown in the userguide example

[mihnita]

To check I did a RuleBasedTransliterator.createFromRules with the rules in the userguide.
The result is indeed "acks".

Checklist

• Required: Issue filed: https://unicode-org.atlassian.net/browse/ICU-21396
• Required: The PR title must be prefixed with a JIRA Issue number. Example: "ICU-1234 Fix xyz"
• Required: The PR description must include the link to the Jira Issue, for example by completing the URL in the first checklist item
• Required: Each commit message must be prefixed with a JIRA Issue number. Example: "ICU-1234 Fix xyz"
• Issue accepted (done by Technical Committee after discussion)
• Tests included, if applicable
• API docs and/or User Guide docs changed or added, if applicable

[markusicu]

@macchiati @poulsbo FYI

[poulsbo]

Age-old typo? Choice of example is to leverage pronunciation similarity, "ax" ~ "acks".