doc: fix description about the latest-codename

fix description about the latest LTS release download page
to make it clear

PR-URL: https://github.com/nodejs/node/pull/4583


Reviewed-By: Rod Vagg <rod@vagg.org>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>