Fix class autoloading issue in ActivityPub::Activity::Create (mastodon#8820)

Author: Gargron

app/lib/activitypub/activity/create.rb

@@ -92,7 +92,7 @@ def process_mention(tag, status)
     return if tag['href'].blank?
 
     account = account_from_uri(tag['href'])
-    account = FetchRemoteAccountService.new.call(tag['href'], id: false) if account.nil?
+    account = ::FetchRemoteAccountService.new.call(tag['href'], id: false) if account.nil?
     return if account.nil?
     account.mentions.create(status: status)
   end