fix typo

shiftregister/app/models.py

@@ -130,7 +130,7 @@ class ShiftRegistration(models.Model):
 
     def can_cancel(self):
         if self.state != self.RegState.REGISTERED:
-            return false
+            return False
         return self.shift.start_at > (
             timezone.now()
             + global_preferences_registry.manager()["helper__min_cancel_time"]