remove reference to dangling from slice::Iter

This aligns the comment with reality and with IterMut. Also dangling does not seem to take any arguments.

library/core/src/slice/iter.rs

@@ -68,7 +68,7 @@ pub struct Iter<'a, T: 'a> {
     ptr: NonNull<T>,
     /// For non-ZSTs, the non-null pointer to the past-the-end element.
     ///
-    /// For ZSTs, this is `ptr::dangling(len)`.
+    /// For ZSTs, this is `ptr::without_provenance_mut(len)`.
     end_or_len: *const T,
     _marker: PhantomData<&'a T>,
 }