Only apply top border radii on first table header row

[harrymander]

Makes tables that have multiple tr elements inside thead look nicer.

E.g. based on index.html, before:

After:

HTML:

<table>
  <thead>
      <tr>
          <th rowspan="2">Feature</th>
          <th colspan="3">CSS framework</th>
      </tr>
      <tr>
          <th>MVP.css</th>
          <th>No CSS</th>
          <th>Custom CSS</th>
      </tr>
  </thead>
 <!-- ... -->
</table>