Fix typo in Email.php
Fix typo in Email.php

Perhaps there should be a test for this
tonyle/sendgrid-php,admiraalmichiel/sendgrid-php,lucasmezencio/sendgrid-php,matthiaz/sendgrid-php,taz77/sendgrid-php,matthiaz/sendgrid-php,tonyle/sendgrid-php,sendgrid/sendgrid-php,taz77/sendgrid-php,ninsuo/sendgrid-php,admiraalmichiel/sendgrid-php,ninsuo/sendgrid-php,steptan/sendgrid-php,lucasmezencio/sendgrid-php,karlpitcher/MeterReadings,sendgrid/sendgrid-php,karlpitcher/MeterReadings,steptan/sendgrid-php