docs: fix link to client secrets format doc

[riccardomurri]

It seems documentation for the "client secrets" file has moved here: https://github.com/googleapis/google-api-python-client/blob/master/docs/client-secrets.md

[google-cla]

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[parthea]

@googlebot I consent.

[parthea]

Superseded by #86