fix doctest

Author: wxtim

cylc/flow/simulation.py

@@ -62,7 +62,7 @@ def __repr__(self):
             >>> this = ModeSettings()
             >>> print(this)
             <flow.simulation.ModeSettings object at ...
-                simulated_run_length  : None
+                simulated_run_length  : 0.0
                 sim_task_fails        : False
         """
         repr_ = [super().__repr__()]