doc: update GOVERNANCE.md to use "meeting chair"

The governance documentation refers to a CTC meeting "moderator" but
that is a confusing term. "Meeting chair" seems more correctly
descriptive. Change instances of "moderator" to "meeting chair".

Refs: https://github.com/nodejs/CTC/issues/23
PR-URL: https://github.com/nodejs/node/pull/9432


Reviewed-By: Rod Vagg <rod@vagg.org>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Josh Gavant <josh.gavant@outlook.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>