Update docs/configuring-playbook-email.md: fix default email sender specified by exim_relay_sender_address

exim_relay_sender_address consists of exim_relay_hostname, which by default is equal to matrix_server_fqn_matrix, whose default value is matrix.example.com

Signed-off-by: Suguru Hirahara <acioustick@noreply.codeberg.org>

docs/configuring-playbook-email.md

@@ -5,7 +5,7 @@ By default, this playbook sets up an [Exim](https://www.exim.org/) email server
 The email server would attempt to deliver emails directly to their final destination.
 This may or may not work, depending on your domain configuration (SPF settings, etc.)
 
-By default, emails are sent from `matrix@example.com` (as specified by the `exim_relay_sender_address` playbook variable).
+By default, emails are sent from `matrix@matrix.example.com`, as specified by the `exim_relay_sender_address` playbook variable.
 
 **Note**: If you are using a Google Cloud instance, [port 25 is always blocked](https://cloud.google.com/compute/docs/tutorials/sending-mail/), so you need to relay email through another SMTP server as described below.