switch dockerhub user from zettio to weaveworks

site/building.md

@@ -54,8 +54,8 @@ $ make
 ```
 
 This will build the weave components and package them into three
-Docker images (`zettio/weave`, `zettio/weavedns`, and
-`zettio/weaveexec`).  These are then exported (as `weave.tar`,
+Docker images (`weaveworks/weave`, `weaveworks/weavedns`, and
+`weaveworks/weaveexec`).  These are then exported (as `weave.tar`,
 `weavedns.tar` and `weaveexec.tar`).
 
 ## Building in a Docker container