looking at the source tree it looks like that's coming from a template:
but on my machine I could not find that file. So I'm not sure if that ends up inside something else or what, exactly. But, I think @javierluraschi was the last one to touch that file so let's send up the bat signal and maybe Javier will drop by to educate us on where the templates end up and whether editing them is possible.
So you could potentially modify that file to fit your needs. Notice that I wouldn't encourage to tweak RStudio internal files, but if you really want to or need to, that's one way. I would also encourage you to open a GitHub issue for us to consider adding a proper file-template editing tool within the UI.