doc: add tniessen to collaborators

PR-URL: https://github.com/nodejs/node/pull/13371


Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>