Docs: use valid severity in example usage

[hugovk]

--severity=notice gives an error message:

$ drush watchdog:delete --severity=notice

In WatchdogCommands.php line 310:

  Unknown severity level: notice.
  Valid severity levels are: Emergency(0), Alert(1), Critical(2), Error(3), Warning(4), Notice(5), Info(6), Debug(7).

--severity=Notice works:

$ drush watchdog:delete --severity=Notice
All messages with severity = 5 will be deleted.

 Do you want to continue? (yes/no) [yes]:
 > yes

 [success] 184 watchdog messages have been deleted.