Fix the link to the Authentication docs

[markvincze]

No description provided.

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[tseaver]

Hmmm, Oops, that was a stray comment.

[tseaver]

Thanks for the patch! I think the preferred / canonical location for these docs is now https://googleapis.dev/python/google-api-core/latest/auth.html

[tseaver]

Superseded by #9187.