One of the wonderful things about steel, a small dent like this is purely of cosmetic concern. I call it a ding if the damage wouldn't be seen or felt on the inside of the tube (not that you can always inspect it from the inside) but even a small dent isn't of significant concern unless it is in a bad spot on the stays. On high end double or triple butted frames, the steel can get pretty thin in spots and dents become more of a concern but on your average touring frame you can safely ride with some pretty gnarly dents in the main tubes. Even if a steel tube eventually fails, the failure isn't usually instantaneous or catastrophic but never say never.

I'd ride that frame without worry and just check the area once in a while.