Update gppControl_usnat.md

[patmmccann]

🏷 Type of documentation

• new bid adapter
• update bid adapter
• new feature
• text edit only (wording, typos)
• bugfix (code examples)
• new examples

📋 Checklist

• Related pull requests in prebid.js or server are linked -> Paste link in this list or reference it on the PR itself
• For new adapters check submitting your adapter docs

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