You're in the right place. And, confusingly, the title is not set in the briefing.html file.
Go into the mission editor and look in the top right hand corner, there's a funny picture of a cloud. Click on it. The official name for this part is the Intel Box. Write the name of your mission here.
The section called Briefing is just for you, any text here doesn't appear anywhere else. Most of the rest is self explanatory, if you need help check my own macguba's guide to mission editing for beginners or any good basic tutorial.