Updated discussion list to stix@mitre.org
Updated discussion list to stix@mitre.org

Updated discussion list to stix@mitre.org and deleted list promo text. Also made a couple of minor copy edits.
johnwunder/johnwunder.github.io,STIXProject/stixproject.github.io,johnwunder/johnwunder.github.io,johnwunder/johnwunder.github.io,johnwunder/johnwunder.github.io,jburns12/stixproject.github.io,STIXProject/stixproject.github.io,jburns12/stixproject.github.io,STIXProject/stixproject.github.io,jburns12/stixproject.github.io,jburns12/stixproject.github.io,STIXProject/stixproject.github.io