update /stats page to use Object

fixes https://console.cloud.google.com/errors/detail/CLq21NfkvMu0WA;time=P30D?project=bridgy-federated

templates/stats.html

@@ -9,7 +9,7 @@
 <li><a href="https://snarfed.org/2017-10-22_bridgy-fed">Launched October 22, 2017</a>
 <li>{{ users }} users</li>
 <li>{{ followers }} fediverse followers</li>
-<li>{{ activities }} activities handled</li>
+<li>{{ objects }} objects handled</li>
 </ul>
 </h3>