Update authentication.mdx to use correct auth header for OpenAI and V…

[ndamulelonemakh]

…ercel SDKs

Using cf-aig-token results in Error: 401 [{"code":2009,"message":"Unauthorized"}]. From my tests this is fixed if we use header name cf-aig-authorization instead

Summary

Update authentication.mdx to use correct auth header for OpenAI and Vercel SDKs

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Documentation checklist

• The documentation style guide has been adhered to.
• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.
• Files which have changed name or location have been allocated redirects.