doc: rename good first contrib label

Now `good first issue`.

PR-URL: https://github.com/nodejs/node/pull/16150
Refs: https://github.com/nodejs/node/issues/16149


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>