Update documentation-guidelines.md
Update documentation-guidelines.md

Fixed typo on line 44 and added "Sub-section Headings..."
kernicPanel/platform,andela/mattermost-platform,tbolon/platform,trashcan/platform,ninja-/platform,cyanlime/platform,ninja-/platform,42wim/platform,cadecairos/mattermoz,rompic/platform,cadecairos/mattermoz,npcode/platform,andela/mattermost-platform,ttyniwa/platform,rompic/platform,rahulltkr/platform,ttyniwa/platform,rgarmsen2295/platform,samogot/platform,rgarmsen2295/platform,florianorben/platform,tbalthazar/platform,samogot/platform,asaadmahmoodspin/platform,42wim/platform,ninja-/platform,ninja-/platform,ttyniwa/platform,andela/mattermost-platform,rgarmsen2295/platform,rahulltkr/platform,ttyniwa/platform,florianorben/platform,npcode/platform,bkeroack/platform,dimitertodorov/platform,trashcan/platform,ninja-/platform,cadecairos/mattermoz,samogot/platform,trashcan/platform,tbolon/platform,ZBoxApp/platform,mozilla/mattermoz,npcode/platform,kernicPanel/platform,rgarmsen2295/platform,npcode/platform,fosstp/platform,daizenberg/platform,asaadmahmoodspin/platform,fosstp/platform,dotcominternet/platform,lawrenae/mattermost,trashcan/platform,mozilla/mattermoz,ZBoxApp/platform,trashcan/platform,mozilla/mattermoz,dotcominternet/platform,asaadmahmoodspin/platform,rahulltkr/platform,florianorben/platform,tbalthazar/platform,lawrenae/mattermost,kernicPanel/platform,ttyniwa/platform,rahulltkr/platform,cyanlime/platform,lawrenae/mattermost,bkeroack/platform,numericube/platform,npcode/platform,lawrenae/mattermost,mozilla/mattermoz,andela/mattermost-platform,daizenberg/platform,cyanlime/platform,mozilla/mattermoz,cadecairos/mattermoz,fosstp/platform,dimitertodorov/platform,andela/mattermost-platform,florianorben/platform,lawrenae/mattermost,cyanlime/platform,ZBoxApp/platform,daizenberg/platform,kernicPanel/platform,tbolon/platform,kernicPanel/platform,rompic/platform,cadecairos/mattermoz,tbalthazar/platform,ZBoxApp/platform,tbolon/platform,bkeroack/platform,dotcominternet/platform,bkeroack/platform,florianorben/platform,rgarmsen2295/platform,numericube/platform,42wim/platform,fosstp/platform,dimitertodorov/platform,numericube/platform,bkeroack/platform,dotcominternet/platform,tbolon/platform,ZBoxApp/platform