Make steps extension only match at start of lines

The steps extension was matching any text starting `s[0-9]+.` rather than only matching at the start of a line. This caused an issue with some HMRC content that contained a sentence ending in a match.

Before this fix this input:

```
Zippy, Bungle and George did not qualify for the tax exemption in s428. They filled in their tax return accordingly.
```

…produced this output:

```
<p>Zippy, Bungle and George did not qualify for the tax exemption in &lt;ol class="steps"&gt;</p>
<li><p>They filled in their tax return accordingly.</p>
</li>
<p>&lt;/ol&gt;</p>
```