doc: add AnnaMag to collaborators

PR-URL: https://github.com/nodejs/node/pull/12414


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Alexey Orlenko <eaglexrlnk@gmail.com>
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>