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>

doc/changelogs/CHANGELOG_V6.md

@@ -7,6 +7,7 @@
 </tr>
 <tr>
 <td valign="top">
+<a href="#6.10.0">6.10.0</a><br/>
 <a href="#6.9.5">6.9.5</a><br/>
 <a href="#6.9.4">6.9.4</a><br/>
 <a href="#6.9.3">6.9.3</a><br/>