Temporarily ignore an example because of a rustdoc bug

Should be fixed in rust 1.35

src/ch04-02-references-and-borrowing.md

@@ -234,7 +234,11 @@ through the last time that reference is used. For instance, this code will
 compile because the last usage of the immutable references occurs before the
 mutable reference is introduced:
 
-```rust,edition2018
+<!-- This example is being ignored because there's a bug in rustdoc making the
+edition2018 not work. The bug is currently fixed in nightly, so when we update
+the book to >= 1.35, `ignore` can be removed from this example. -->
+
+```rust,edition2018,ignore
 let mut s = String::from("hello");
 
 let r1 = &s; // no problem