fix usage comments on boostd data-transfers commands

[dirkmc]

$ ./boostd data-transfers cancel --help
NAME:
   boostd data-transfers cancel - Force cancel a data transfer

USAGE:
   boostd data-transfers cancel [command options] <transfer id>

OPTIONS:
   --peerid value          narrow to transfer with specific peer
   --initiator             specify only transfers where peer is/is not initiator (default: false)
   --cancel-timeout value  time to wait for cancel to be sent to client (default: 5s)
   --help, -h              show help