Improve wording for StepBy

No other iterator makes the distinction between an iterator and an iterator adapter
in its summary line, so change it to be consistent with all other adapters.

Author: steveklabnik

src/libcore/iter/mod.rs

@@ -546,7 +546,7 @@ impl<I> Iterator for Cycle<I> where I: Clone + Iterator {
 #[unstable(feature = "fused", issue = "35602")]
 impl<I> FusedIterator for Cycle<I> where I: Clone + Iterator {}
 
-/// An adapter for stepping iterators by a custom amount.
+/// An iterator for stepping iterators by a custom amount.
 ///
 /// This `struct` is created by the [`step_by`] method on [`Iterator`]. See
 /// its documentation for more.