Change UseVPC to DdUseVPC in PrivateLink documentation

[Telling]

What does this PR do?

Fixes documentation for the PrivateLink configuration, as I ran into this today. Also mentioned in #636.

Types of changes

• Bug fix
• New feature
• Breaking change
• Misc (docs, refactoring, dependency upgrade, etc.)

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• This PR's description is comprehensive
• This PR contains breaking changes that are documented in the description
• This PR introduces new APIs or parameters that are documented and unlikely to change in the foreseeable future
• This PR impacts documentation, and it has been updated (or a ticket has been logged)
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