corrected heading for data model documentation
corrected heading for data model documentation

STIXProject/stixproject.github.io,johnwunder/johnwunder.github.io,STIXProject/stixproject.github.io,jburns12/stixproject.github.io,STIXProject/stixproject.github.io,jburns12/stixproject.github.io,jburns12/stixproject.github.io,johnwunder/johnwunder.github.io,STIXProject/stixproject.github.io,johnwunder/johnwunder.github.io,jburns12/stixproject.github.io,johnwunder/johnwunder.github.io