Add preferences screenshot (#727)

Author: int128

README.md

@@ -18,11 +18,11 @@ Here are the screenshots generated by Storycap in GitHub Actions.
   </tr>
   <thead>
     <th>Bookmark Editor</th>
-    <th></th>
+    <th>Preferences</th>
   </thead>
   <tr>
     <td><img src="https://github.com/int128/bntp/wiki/main/__screenshots__/BookmarkEditor/component/Primary.png"></td>
-    <td></td>
+    <td><img src="https://github.com/int128/bntp/wiki/main/__screenshots__/Preferences/component/Primary.png"></td>
   </tr>
 </table>