Remove some padding from code blocks

This creates a weird "leading indent" on the first line of code blocks, at least in Chrome 46, and ends up making the first row not line up nicely with everything else.

![Screen_Shot_2015-10-14_at_2.40.38_PM](https://gitlab.com/pioto/gitlab-ce/uploads/5434eb077c17e56528a94079afe3c732/Screen_Shot_2015-10-14_at_2.40.38_PM.png)


See merge request !1596