Fix property name typo in PhabricatorPDFIterator

Summary: $framgentKey has a typo.

Test Plan: Slowly spell the word "fragmentKey"; compare with the top of the file.

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/D25715