cassiopeia: change link to invoice to be relative

apps/cassiopeia/README.md

@@ -6,7 +6,7 @@ The kookie office ecosystem is a set of free software, plain text
 tools that I use to run my business.  The other tool you may want to
 look at is called [invoice]!
 
-[invoice]: https://git.spacekookie.de/kookienomicon/tree/infra/libkookie/overlays/kookie/invoice
+[invoice]: ../../infra/libkookie/overlays/kookie/invoice
     
 ## How to use