Fix reference for tts:fontVariant derivation

Author: palemieux

spec/ttml2.xml

@@ -28334,7 +28334,7 @@ width and height independently is an extension introduced by TTML.</p></item>
 <tbody>
 <tr>
 <td><phrase role="strong">Reference</phrase></td>
-<td><bibref ref="css3-fonts"/>, &sect;6.11</td>
+<td><bibref ref="css3-fonts"/>, &sect;6</td>
 </tr>
 <tr>
 <td><phrase role="strong">Model</phrase></td>