docs: add sender email to example

Author: fulleni

README.md

@@ -50,6 +50,7 @@ void main() async {
   // 3. Use the client to send an email
   try {
     await emailClient.sendTransactionalEmail(
+      senderEmail: '[email protected]',
       recipientEmail: '[email protected]',
       templateId: 'd-your-template-id',
       templateData: {'name': 'World'},