Fix broken link to 'Building Weave Net' documentation.

test/README.md

@@ -18,7 +18,7 @@ To meet the aforementioned ssh requirement you may want to
 
 ## Running tests
 
-If you are [building weave using Vagrant](http://docs.weave.works/weave/latest_release/building.html),
+If you are [building weave using Vagrant](https://www.weave.works/docs/net/latest/building/),
 it is recommended to run the tests from the build VM and not the host.
 
     ./setup.sh