doc: revise TSC text in GOVERNANCE.md

Tighten up a few things to make them more concise. Clarify that changes
to the charter require board approval.

PR-URL: https://github.com/nodejs/node/pull/27169


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Beth Griggs <Bethany.Griggs@uk.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>