doc: dns.lookup() documentation error code

dns.lookup() will always return ENOTFOUND on a empty lookup
or when the host not found.
https://github.com/nodejs/node/blob/master/lib/internal/errors.js#L503

Fixes: https://github.com/nodejs/node/issues/27604

PR-URL: https://github.com/nodejs/node/pull/27625


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