Helper example still uses v5 api

[enugentdt]

Issue Summary

The helpers example README still follows v5 api conventions, and has not updated. I'm including a PR to fix this.

For example:

 mail = Mail(from_email, subject, to_email, content)

should become:

 mail = Mail(from_email, to_email, subject, content)

The above was in the README as an example.

Also, I updated the last code block:
From:

    sg = SendGridAPIClient()
    data = build_kitchen_sink()
    response = sg.client.mail.send.post(request_body=data)

To:

    sg = SendGridAPIClient()
    data = build_kitchen_sink()
    response = sg.send(data)