message: fix typo in command args
diff: @@ -12,7 +12,7 @@ Then, the certificates of trusted users should be registered in CCF via the memb
  .. code-block:: bash
  - $ memberclient --cert member1_cert --privk member1_privk --rpc-address rpc_ip:rpc_port --ca network_cert add_user --user_cert user_cert
  + $ memberclient --cert member1_cert --privk member1_privk --rpc-address rpc_ip:rpc_port --ca network_cert add_user --user-cert user_cert
  {"commit":4,"global_commit":3,"id":0,"jsonrpc":"2.0","result":{"completed":false,"id":0},"term":2}
  Other members are then allowed to vote for the proposal, using the proposal id returned to the proposer member (here ``0``, as per ``"result":{"completed":false,"id":0}``).
  