Fixed a broken link

src/doc/rustc-dev-guide/src/traits-canonicalization.md

@@ -41,7 +41,7 @@ trait query: `?A: Foo<'static, ?B>`, where `?A` and `?B` are unbound.
 This query contains two unbound variables, but it also contains the
 lifetime `'static`. The trait system generally ignores all lifetimes
 and treats them equally, so when canonicalizing, we will *also*
-replace any [free lifetime](./background.html#free-vs-bound) with a
+replace any [free lifetime](./appendix-background.html#free-vs-bound) with a
 canonical variable. Therefore, we get the following result: 
 
     ?0: Foo<'?1, ?2>