Update traffic-transformation.md to change whitelist ref

[brianoh1979]

Change reference from *whitelist plugin to Allow List plugin for https://tyk.io/docs/api-management/traffic-transformation/#api-designer-24

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Checklist

• Added a preview link
• Reviewed AI PR Agent suggestions
• For Tyk Members - Added Jira DX PR ticket to the subject
• For Tyk Members - Added the appropriate release labels (for fixes add the latest release)

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