Fix user_name parameter typo in upgrading to API keys doc

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Description of the change: The SMTP settings in the upgrade docs should be

user_name: "apikey"
password: <Your API Key>

but the docs reference the user_name as username which is wrong.

Reason for the change: If you implement as per the current doc, you get ArgumentError SMTP-AUTH requested but missing user name

Link to original source: https://sendgrid.com/docs/for-developers/sending-email/upgrade-your-authentication-method-to-api-keys/

Closes #6464

sendgrid / docs