Fix doc rename struct to class (#2648)

Fix class declaration on the doc

docs/design/generics/appendix-coherence.md

@@ -67,7 +67,7 @@ this:
 
     ```
     package Container;
-    struct HashSet(Key:! Hashable) { ... }
+    class HashSet(Key:! Hashable) { ... }
     ```
 
 -   A `Song` type is defined in package `SongLib`.