fixup! Add backgroundSegmentationMask to VideoFrameMetadata registry

Update URL.

Author: eehakkin

video_frame_metadata_registry.src.html

@@ -79,7 +79,7 @@
   </tr>
   <tr>
     <td>backgroundSegmentationMask</td>
-    <td>[Background segmentation mask](https://github.com/w3c/mediacapture-extensions/pull/142)</td>
+    <td>[Background segmentation mask](https://w3c.github.io/mediacapture-extensions/#background-segmentation-mask)</td>
   </tr>
 </table>