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