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