doc: add missing entry in v6 changelog table

This adds an anchor for v6.10.0 in the LTS column.

PR-URL: https://github.com/nodejs/node/pull/11534


Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>