Fix missing parameter in call to display function

We don't use the __str__() call very often on volunteer slots, but we do
when generating notifications from API operations...

postgresqleu/confreg/models.py

@@ -1281,7 +1281,7 @@ class Meta:
         ordering = ['timerange', ]
 
     def __str__(self):
-        return self._display_timerange()
+        return self._display_timerange(None)
 
     def _display_timerange(self, cache):
         start = timezone.localtime(self.timerange.lower)