docs(README): add missing subject parameter in email example

- Added the 'subject' parameter to the sendTransactionalEmail function in the README.md example
- This change improves the clarity and completeness of the example, ensuring users have all necessary information to send an email

Author: fulleni

README.md

@@ -52,6 +52,7 @@ void main() async {
     await emailClient.sendTransactionalEmail(
       senderEmail: '[email protected]',
       recipientEmail: '[email protected]',
+      subject: 'Your Subject Here',
       templateId: 'd-your-template-id',
       templateData: {'name': 'World'},
     );