Fix broken link in proposal text (#948)

proposals/p0553.md

@@ -158,8 +158,9 @@ external impl Comparable for Song { ... }
 ```
 
 The main advantage of this syntax was that it was uniform across many cases,
-including [conditional conformance](details.md#conditional-conformance). It
-wasn't ideal across a number of dimensions though.
+including
+[conditional conformance](/docs/design/generics/details.md#conditional-conformance).
+It wasn't ideal across a number of dimensions though.
 
 -   It repeated the type name which was redundant and verbose
 -   It could affect the API of the type outside of the type definition.