doc: fix confusing example in dns.md

Currently, the example throws 'Error: getHostByAddr ENOTFOUND'
The previous example is edited for consistency.

PR-URL: https://github.com/nodejs/node/pull/11022


Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>