Incorrect information about producer:close in README

[Satbek]

local err = producer:close() -- always stop consumer to send all pending messages before app close
if err ~= nil then
     print(err)
     os.exit(1)
end

but producer:close returns true, nil or false, err

function Producer:close()
    local ok, err = self._producer:close()

    self._msg_delivery_poll_fiber:cancel()
    if self._poll_logs_fiber ~= nil then
        self._poll_logs_fiber:cancel()
    end
    if self._poll_errors_fiber ~= nil then
        self._poll_errors_fiber:cancel()
    end

    self._producer = nil

    return ok, err
end