doc(library): Fix Markdown in Iterator::by_ref.

This patch fixes the Markdown formatting in
`std::core::iter::Iterator::by_ref`. Code is used inside a link without
the backticks around the code.

Author: Hywan

library/core/src/iter/traits/iterator.rs

@@ -1875,7 +1875,7 @@ pub trait Iterator {
     /// without giving up ownership of the original iterator,
     /// so you can use the original iterator afterwards.
     ///
-    /// Uses [impl<I: Iterator + ?Sized> Iterator for &mut I { type Item = I::Item; ...}](https://doc.rust-lang.org/nightly/std/iter/trait.Iterator.html#impl-Iterator-for-%26mut+I).
+    /// Uses [`impl<I: Iterator + ?Sized> Iterator for &mut I { type Item = I::Item; ...}`](https://doc.rust-lang.org/nightly/std/iter/trait.Iterator.html#impl-Iterator-for-%26mut+I).
     ///
     /// # Examples
     ///