Fix searching legalpad documents by contributors

Summary:
Found in a change submitted to Phorge (https://we.phorge.it/D25018#inline-231), this fixes a typo in populating search the contributorPHIDs constraint in the Legalpad search engine.

Currently when trying to search legalpad documents by contributor an error is encountered:

Array for %Ls conversion is empty. Query: contributor.dst IN (%Ls)

Test Plan: I searched for legalpad documents based on a contributor and got back correct results.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D21719