Fix count usize link typo in docs

src/libcore/iter/iterator.rs

@@ -168,7 +168,7 @@ fn size_hint(&self) -> (usize, Option<usize>) { (0, None) }
     /// This function might panic if the iterator has more than [`usize::MAX`]
     /// elements.
     ///
-    /// [`usize::MAX`]: ../../std/isize/constant.MAX.html
+    /// [`usize::MAX`]: ../../std/usize/constant.MAX.html
     ///
     /// # Examples
     ///