add whitespace before current target to avoid header overlap

This is one of the nagging items left on my asana so I figured I'd do a quick fix. Like the comment says, this is an imperfect trade off but I think it is worth it. The alternative would probably be to do various javascript events to watch for scroll, load, and hash link navigation and add an offset there but that just strikes me as a lot of bug prone hassle for a minor problem.

This instead trades one minor problem - being forced to scroll up to see text in certain situations - with another IMO more minor problem - a wee bit more whitespace visible (via the height and the non-breaking space in content) if you do scroll up in those certain situations.

I've now spent more time writing this message than the actual solution so if you don't care for it, just go ahead and close this PR but I think it is worth a merge.