Fix a constant typo in Diviner ("DECLARATAION" -> "TION")

Summary: Fixes T12351. This got typo'd in D17377.

Test Plan: bin/diviner generate --clean --book src/docs/book/phabricator.book

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12351

Differential Revision: https://secure.phabricator.com/D17460