Fixes padding in multi-line code blocks

`padding: 5px 12px;` on `pre` makes for balanced horizontal and vertical padding on monospace fonts; `padding: 0;` on `pre code` fixes the first line of a code block being indented by the `code` padding rule while the rest aren't.

![changes](https://f.cloud.github.com/assets/1760908/170680/c584ad90-7a78-11e2-8d12-4e506066df3e.png)