Fix issue with syntax highlighting + line nums.

Below is the generated HTML for with line numbers and without line numbers, respectively.

```html
<!-- Without line numbers -->
<pre class="sourceCode scala">
    <code class="sourceCode scala">
        <span class="kw">trait</span>
        Monad[M[_]]
    </code>
</pre>

<!-- With line numbers -->
<table class="sourceCode scala numberLines">
    <tr class="sourceCode">
        <td class="lineNumbers">
            <pre>1</pre>
        </td>
        <td class="sourceCode">
            <pre>
                <code class="sourceCode scala">
                    <span class="kw">trait</span>
                    Monad[M[_]]
                </code>
            </pre>
        </td>
    </tr>
</table>

```

The markdown used is:

```

~~~~ { .scala }
trait Monad[M[_]]
~~~~

~~~~ { .scala .numberLines}
trait Monad[M[_]]
~~~~

```

Notice that without line numbers the outer `pre` has a `sourceCode` class whereas with line numbers the `sourceCode` class is only put in `code` and table-related tags.

In the original `syntax.css` file synax highlighting was only triggered if the `kw` and friends tags were within a `pre.sourceCode` - that means it did not trigger with line numbers. This change removes the requirement for the `pre` prefix so that highlighting is triggered with or without line numbers, so long as the code is within *some* `.sourceCode` tag.