Its funny, because it actually didn't have this. (Or, literally this)

https://skeptics.stackexchange.com/questions/28476/have-the-ending-credits-of-rambo-iii-been-changed

Original sources from the time of the movie's release, 1988, state that the film is dedicated to the "gallant people of Afghanistan".

The edited end-card was made as a joke, back in 2004, and circulated so many times online that it eventually became headcannon.