docs: Update mail references to message

[Stancobridge]

This PR updates the documentation to replace instance of mail with message for consistency with the documentation and to avoid confusion.

The affected line was:

"You may define the email subject using the message.subject method and the email's sender using the mail.from method."

It has been updated to:

"You may define the email subject using the message.subject method and the email's sender using the message.from method."