Minor fix for JsonLintJsonLinter class.
Summary: Remove a \ character, which will probably cause issues on PHP versions earlier than 5.3.
Test Plan: Thought long and hard.
Reviewers: epriestley, #blessed_reviewers
Reviewed By: epriestley, #blessed_reviewers
Subscribers: epriestley, Korvin
Differential Revision: https://secure.phabricator.com/D9636