advise user to replace test@example.com with their verified Sender Identity

[aaronkelton]

Using the default values supplied can confuse a new user why their email isn't getting sent. As supplied, the response returns an error:

The from address does not match a verified Sender Identity. Mail cannot be sent until this error is resolved. Visit https://sendgrid.com/docs/for-developers/sending-email/sender-identity/ to see the Sender Identity requirements

#<SendGrid::Response:0x000000010aaa22c8 @status_code="403", @body="{\"errors\":[{\"message\":\"The from address does not match a verified Sender Identity. Mail cannot be sent until this error is resolved. Visit https://sendgrid.com/docs/for-developers/sending-email/sender-identity/ to see the Sender Identity requirements\",\"field\":\"from\",\"help\":null}]}", @headers={"server"=>["nginx"], "date"=>["Sun, 17 Jul 2022 20:06:44 GMT"], "content-type"=>["application/json"], "content-length"=>["281"], "connection"=>["close"], "access-control-allow-origin"=>["https://sendgrid.api-docs.io"], "access-control-allow-methods"=>["POST"], "access-control-allow-headers"=>["Authorization, Content-Type, On-behalf-of, x-sg-elas-acl"], "access-control-max-age"=>["600"], "x-no-cors-reason"=>["https://sendgrid.com/docs/Classroom/Basics/API/cors.html"], "strict-transport-security"=>["max-age=600; includeSubDomains"]}>

Fixes

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