R1:8f2f841d172b

Fix some links to "Adding New Classes" in docs

Summary: Fixes T9483. This bookname is `phabcontrib`, not `contributor`.

Test Plan: `grep` / clicked these links.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9483

Differential Revision: https://secure.phabricator.com/D14195