Add missing push notification title for polls (#15929)

config/locales/en.yml

@@ -1044,6 +1044,8 @@ en:
       body: 'You were mentioned by %{name} in:'
       subject: You were mentioned by %{name}
       title: New mention
+    poll:
+      title: A poll has ended
     reblog:
       body: 'Your status was boosted by %{name}:'
       subject: "%{name} boosted your status"