Fix toc with one element

[sasanikolic90]

The TOC structure is wrongly built when there is only one heading in the toc scope. I think the second ul tag is not necessary.

That is caused by getTopLevel() function, that in this case returns 1 as the top level.

[sasanikolic90]

Created a PR here: #55

Not sure if this is actually correct, since I changed some core functionality here. In fact, I totally opted out h1 titles and made the top level default to 2. This fixes the issue.