No. I clearly remember seeing that in the end credits, when I first watched Rambo 3 on TV, in the late 90s. Rambo 3 was a great movie and I liked it more than the first one, because I was a kid back then.
I remember it clearly because that was the first movie I saw exploding arrows. I don’t know if the explosive arrows were made famous in Rambo 3 or in Predator. In any case I watched quite a few reruns of Rambo 3 on TV, and they clearly had that dedicated to mujahideen fighters message in the end credits.
Not a hoax. This was on the VHS release of Rambo. "The dedication on the VHS release of the film said "to the brave Mujahideen fighters". It was later changed to "to the gallant people of Afghanistan"." See https://en.wikipedia.org/wiki/Rambo_III