fix an old python-leap url

Author: hjwp

chapter_01_domain_model.asciidoc

@@ -303,7 +303,7 @@ class Batch:
     `typing.Optional` and `datetime.date`. If you want to double-check
     anything, you can see the full working code for each chapter in
     its branch (e.g.,
-    https://github.com/python-leap/code/tree/chapter_01_domain_model[chapter_01_domain_model]).
+    https://github.com/cosmicpython/code/tree/chapter_01_domain_model[chapter_01_domain_model]).
 
 <3> Type hints are still a matter of controversy in the Python world. For
     domain models, they can sometimes help to clarify or document what the