* Update notify.mailgun.markdown
Fix inconsistency where "domain" is required in the Mailgun Component https://www.home-assistant.io/components/mailgun/ but "sandbox" can also be used to specify the domain. It appears that the package https://github.com/pschmitt/pymailgunner in def guess_domain(...) allows multiple domains and "sandbox" can be used to choose a sandbox domain over a normal domain. As far as I can tell, this feature isn't used by the Mailgun component. Also removed all references to the obsolete word "token" to avoid confusion. Also used a different name for the notify service to differentiate it from the mailgun component. Note: There are also updates to the aforementioned Mailgun Component doc.
* There is no need for the postfix
