fixed small typo

was

```
<h1 class="section-h1"><a href="#">Section h1</a></h1>
<h2 class="section-h2"><a href="#">Section h2</a></h2>
<h3 class="section-h3"><a href="#">Section h3</a></h3>
<h4 class="section-h3"><a href="#">Section h4</a></h4> 
```
changed to

```
<h1 class="section-h1"><a href="#">Section h1</a></h1>
<h2 class="section-h2"><a href="#">Section h2</a></h2>
<h3 class="section-h3"><a href="#">Section h3</a></h3>
<h4 class="section-h4"><a href="#">Section h4</a></h4>
```