PEP 785: Fix typo in parameter name for leaf_exceptions function (#5122)

Author: Darsstar

peps/pep-0785.rst

@@ -197,7 +197,7 @@ A ``leaf_exceptions()`` helper function
 
 
     def leaf_exceptions(
-        self: BaseExceptionGroup, *, fix_traceback: bool = True
+        self: BaseExceptionGroup, *, fix_tracebacks: bool = True
     ) -> list[BaseException]:
         """
         Return a flat list of all 'leaf' exceptions.