doc: add Leko to collaborators

PR-URL: https://github.com/nodejs/node/pull/18117


Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>