message: update doc of the blockchain log help message
diff: @@ -398,7 +398,7 @@ fn blockchain_commands_definition<'a, 'b>() -> App<'a, 'b> {
  .arg(blockchain_argument_name_definition())
  )
  .subcommand(SubCommand::with_name("log")
  - .about("print some details about the given blockchain")
  + .about("print the block, one by one, from the given blockhash or the tip of the blockchain.")
  .arg(blockchain_argument_name_definition())
  .arg(Arg::with_name("HASH_BLOCK")
  .value_name("HASH")
  