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