Diviner: Fix a broken link in "Adding New Classes" article
Summary: There is no codebase called "phabricator" anymore.
Test Plan:
# Go to https://we.phorge.it/book/contrib/article/adding_new_classes/ or http://phorge.localhost/book/contrib/article/adding_new_classes/ and click on the first link in the text ("Phabricator Application").
# Run `./bin/diviner generate`
# Repeat step 1
Reviewers: O1 Blessed Committers, mainframe98
Reviewed By: O1 Blessed Committers, mainframe98
Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Differential Revision: https://we.phorge.it/D26257