Add MTCaptcha to third-parties in privacy policy

MTCaptcha does replace reCAPTCHA for MusicBrainz account registration.

Reference: https://tickets.metabrainz.org/browse/MBS-10039

Author: yvanzo

metabrainz/templates/index/privacy.html

@@ -76,6 +76,9 @@ <h3>{{ _('Third-Party content') }}</h3>
     <li>
       reCAPTCHA: Our account creation page loads third party content in order to prevent spammers from creating accounts.
     </li>
+    <li>
+      MTCaptcha: Our account creation page loads third party content in order to prevent spammers from creating accounts.
+    </li>
   </ol>
 
   <p>