fix: rename tab label from "Mention" to "Mentions" (#2570)

locales/en.json

@@ -632,7 +632,7 @@
     "notifications_favourite": "Favourite",
     "notifications_follow": "Follow",
     "notifications_follow_request": "Follow request",
-    "notifications_mention": "Mention",
+    "notifications_mention": "Mentions",
     "notifications_more_tooltip": "Filter notifications by type",
     "notifications_poll": "Poll",
     "notifications_reblog": "Boost",