doc: add table header in intl.md

An empty header element in a table is an accessibility issue. In this
case, specifying the column as containing a feature seems to be the way
to go.

PR-URL: https://github.com/nodejs/node/pull/36261


Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>