Use @example.com instead of @sample.com

[yepher]

Domains like sample.com and test.com is owned by someone. Placeholder email address examples should use @example.com instead.

See:
https://github.com/SparkPost/php-sparkpost/blob/master/examples/transmission/rfc822.php#L15
https://github.com/SparkPost/php-sparkpost/blob/master/examples/transmission/send_transmission_all_fields.php#L21
https://github.com/SparkPost/php-sparkpost/blob/master/examples/transmission/stored_template_send.php#L16

[beardyman]

@yepher - Yup, I didn't notice those before but I already implemented a change in the rework that I have in progress now (http-adapter-update branch): https://github.com/SparkPost/php-sparkpost/blob/http-adapter-update/examples/transmission/send_transmission_all_fields.php

[beardyman]

closing. fixed in #22