Modified the smtp server tutorial according to http://twistedmatrix.com/trac/ticket/3324#comment:8 and http://twistedmatrix.com/trac/ticket/3324#comment:9

git-svn-id: svn://svn.twistedmatrix.com/svn/Twisted/branches/smtp-server-tutorial-3324@35871 bbbe8e31-12d6-0310-92fd-ac37d47ddeeb