fix a comment in std::iter::successors

The `unfold` function have since been renamed to `from_fn`.

library/core/src/iter/sources/successors.rs

@@ -17,7 +17,7 @@ where
     F: FnMut(&T) -> Option<T>,
 {
     // If this function returned `impl Iterator<Item=T>`
-    // it could be based on `unfold` and not need a dedicated type.
+    // it could be based on `from_fn` and not need a dedicated type.
     // However having a named `Successors<T, F>` type allows it to be `Clone` when `T` and `F` are.
     Successors { next: first, succ }
 }