Add missing quotes on Mailbox test template

actionmailbox/lib/rails/generators/test_unit/templates/mailbox_test.rb.tt

@@ -5,7 +5,7 @@ require "test_helper"
 class <%= class_name %>MailboxTest < ActionMailbox::TestCase
   # test "receive mail" do
   #   receive_inbound_email_from_mail \
-  #     to: '"someone" <someone@example.com>,
+  #     to: '"someone" <someone@example.com>',
   #     from: '"else" <else@example.com>',
   #     subject: "Hello world!",
   #     body: "Hello?"