Fix a broken link to Pair::as_span

Author: jieyouxu

pest/src/span.rs

@@ -18,7 +18,7 @@ use crate::position;
 /// A span over a `&str`. It is created from either [two `Position`s] or from a [`Pair`].
 ///
 /// [two `Position`s]: struct.Position.html#method.span
-/// [`Pair`]: ../iterators/struct.Pair.html#method.span
+/// [`Pair`]: ./iterators/struct.Pair.html#method.as_span
 #[derive(Clone, Copy)]
 pub struct Span<'i> {
     input: &'i str,