Fix help messages for update commands

[Hiroaki Kobayashi]

The help message of "climate lease-update" command is wrong.
The word "delete" in the message shown below should be "update".

-----help message--------------------------------------------------------------------
$ climate help lease-update
usage: climate lease-update [-h] [--name NAME]
                            [--prolong-for PROLONG_FOR | --reduce-by REDUCE_BY]
                            [--defer-by DEFER_BY | --advance-by ADVANCE_BY]
                            LEASE

Update a lease.

positional arguments:
  LEASE ID or name of lease to delete <-!!!HERE!!! "delete" should be "update"

optional arguments:
  -h, --help show this help message and exit
  --name NAME New name for the lease
  --prolong-for PROLONG_FOR
                        Time to prolong lease for
  --reduce-by REDUCE_BY
                        Time to reduce lease by
  --defer-by DEFER_BY Time to defer the lease start
  --advance-by ADVANCE_BY
                        Time to advance the lease start
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