Fix auth header s/authentication/authorization

[magec]

I was blindly copying http headers from reference and the server kept saying 'Permission denied'

{
 "type": "/std-errors",
 "status": 403,
 "title": "Access Denied",
 "detail": "action not allowed",
 "details": "action not allowed"
}

Then I realized is authorization not authentication (by reverse engeniering kumactl XD), change it and worked well. This change updates the doc.

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[lahabana]

Thx for the contrib!