Fix typo "pohrge" in "Adding New Classes" documentation

Summary: Fix typo "pohrge" in "Adding New Classes" documentation

Test Plan: Carefully read letter by letter.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25541