Fixed username links in the performance guide
These would end up being rendered as:
<a href="...">@yorickpeterse</a>
<a href="...">@yorickpeterse</a>
[ci skip]
These would end up being rendered as:
<a href="...">@yorickpeterse</a>
<a href="...">@yorickpeterse</a>
[ci skip]