Fix header for docs/concepts/structure.md
Fix header for docs/concepts/structure.md

Markdown didn't have proper spacing for the header, resulting in improper output on the actual site.
driftyco/ionic-site,driftyco/ionic-site,ionic-team/ionic-site,driftyco/ionic-site,ionic-team/ionic-site,driftyco/ionic-site,ionic-team/ionic-site,ionic-team/ionic-site,driftyco/ionic-site