doc: reduce spacing of table of contents.

A margin for the top level list was leaking into nested ones.
auto merge of #15387 : huonw/rust/toc-space, r=alexcrichton

A margin for the top level list was leaking into nested ones.

before; after:

![screenshot from 2014-07-04 08 18 32](https://cloud.githubusercontent.com/assets/1203825/3476459/0b7f8ea8-0300-11e4-8b67-0c47b7931d56.png)