tools: fix gitignore for tools/doc/

PR-URL: https://github.com/nodejs/node/pull/17224
Fixes: https://github.com/nodejs/node/issues/17216


Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>