Fix a typo in documentation

Summary: Platypus vs Platyplus. This was reported downstream in https://phabricator.wikimedia.org/T380365

Test Plan: Read slowly.

Reviewers: O1 Blessed Committers, avivey

Reviewed By: O1 Blessed Committers, avivey

Subscribers: avivey, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25837