Allow non-example code blocks to be scrolled.

## Problem

Look at the pre formatted block in the Error Handling section of the [EVAL command documentation](http://redis.io/commands/eval).  It has a long line in the `pre` code block that gets clipped, without the ability to see the rest of the long line.  The clipping is happening on the `div.text` tag, which has the `overflow: hidden` property.

## Solution

Make the `pre` tag scrollable using the `overflow: auto` property.

The `div.example` block already has the `overflow: auto` property, so `pre` tags within it shouldn't be scrollable or clipped, so I avoided that with the `overflow: visible` property.