message: Change `changeTopic` to `gotoThread` in readme
  Hi,
  I guess `changeTopic` is an old function and it has been changed to `gotoThread`
  So this PR update it in the readme.md
diff: @@ -738,19 +738,19 @@ bot.createConversation(message, function(err, convo) {
  {
  pattern: 'yes',
  callback: function(response, convo) {
  - convo.changeTopic('yes_thread');
  + convo.gotoThread('yes_thread');
  },
  },
  {
  pattern: 'no',
  callback: function(response, convo) {
  - convo.changeTopic('no_thread');
  + convo.gotoThread('no_thread');
  },
  },
  {
  default: true,
  callback: function(response, convo) {
  - convo.changeTopic('bad_response');
  + convo.gotoThread('bad_response');
  },
  }
  ]);
  