Dear [name of manager],
I always dine at your restaurant because of the excellent food and awesome service. I know no restaurant is perfect because there was this one time when I went there and the food I ordered was cold. It is not something I am used to though so I would like to point this out with the hopes of it never happening again.
Take not that this is not something I will post on social media. I pointed it out for your benefit so you can do something to prevent it from happening in the future. When you do, I will be glad to go back to your restaurant over and over again.