fix confusing parameter information

Summary: reference to #10710

Test Plan:
// Add a bitcoin-abc node firstly
bitcoin-cli addnode 67.82.89.67 add

// Commands and returned results as following:

bitcoin-cli getaddednodeinfo true

   error code: -24
   error message:
   Error: Node has not been added.

bitcoin-cli getaddednodeinfo true 67.82.89.67

  error code: -1
  error message:
  getaddednodeinfo ( "node" )

// the correct usage:
bitcoin-cli getaddednodeinfo
bitcoin-cli getaddednodeinfo 67.82.89.67

So getaddednodeinfo only have one optional argument<node>.

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix, qshuai

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Subscribers: jasonbcox, deadalnix, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1447