Sorry, Serling did not narrate any Nostradamus specials. I remember one called “The Man Who Saw Tomorrow” that was narrated by Orson Welles, another famous voice/figure, so perhaps that explains the confusion.
My other guess is that this refers not to a special entirely on Nostradamus but one that might have included a segment on him. Rod narrated several documentaries that covered extraordinary material and could have mentioned Nostradamus. Shows such as:
UFOs: It Has Begun (1976)
The Outer Space Connection (1975)
In Search of Ancient Mysteries (1975)
Monsters! Mysteries or Myths? (1974)
UFOs: Past, Present, and Future (1974)
In Search of Ancient Astronauts (1973)
The Unexplained (1970)