updated link to experimental implementation branch

Author: iritkatriel

except_star.md

@@ -740,7 +740,7 @@ and modify the traceback display code.
   https://github.com/python/exceptiongroups/issues/3#issuecomment-716203284
 
 * A WIP implementation of the `ExceptionGroup` type by @iritkatriel
-  tracked [here](https://github.com/iritkatriel/cpython/tree/exceptionGroup).
+  tracked [here](https://github.com/iritkatriel/cpython/tree/exceptionGroup-stage4).
 
 * The issue where this concept was first formalized:
   https://github.com/python/exceptiongroups/issues/4