Fix nested list margins in auto-generated [TOC]

https://bitbucket.org/miloslevacic/bitbucket-bug-markdown-toc

Here's a public repo with just a README.md that demonstrates the problem.

The top margins for the nested <ul> elements are way off, so the auto-generated [TOC] listing looks weird.

I haven't tested it, but I assume the same bug would be present when just creating nested lists manually (I wanted to reproduce the bug here in the issue report, but it doesn't show up due to a different stylesheet).