Fix error in AdminMailer#new_pending_account (mastodon#10264)

Author: Gargron

app/mailers/admin_mailer.rb

@@ -15,8 +15,8 @@ def new_report(recipient, report)
     end
   end
 
-  def new_pending_account(recipient, account)
-    @account  = account
+  def new_pending_account(recipient, user)
+    @account  = user.account
     @me       = recipient
     @instance = Rails.configuration.x.local_domain