usage message doesn't include ledger_cleaner (Version: 1.5 beta 6)

[carlhua]

The commandline interface usage output (rippled --help) doesn't include the ledger_cleaner command, even though that command works.

I'm not sure how to set any of the parameters of the ledger_cleaner from the commandline, but it returns the ledger-cleaner specific "Cleaner configured" response when you I run the command:

rippled -- ledger_cleaner

Please add the usage information for this command to rippled's --help output.

Original reporter @mDuo13 - RIPD-915