R1:4f2bca58fcea

Fix typo in diviner user guide / diffusion

Summary: Fixes T12445. Reads better.

Test Plan: Read it a few more times.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T12445

Differential Revision: https://secure.phabricator.com/D17546