I've heard of the dry ice approach, tho' I've only seen vid's of it done on bigger dents than what yours appears as... You can buy dry ice at some local grocery stores, you just might have to ask where it is located, and handle with care... I've worked as a research chemist and seen people who knew better get cold burns from improper handling... As its "temp" is around -100F. Without proper, heavy gloves you can freeze your skin = frostbite.
The idea with dry ice is that metal will contract when cold, so a dent can simply "tighten" itself into being flat again.
Ive also seen a youtube video where a person did the same thing with a can of compressed air... They turned the can upside down and sprayed the dent, with the metal cooling again, until it became taut and "knocked" itself out. YouTube - How to Remove Dents with Dry Ice