Fix the ArcanistJSONLintLinter::getVersion function.

Summary: Ref T4954. There was a typo in the original implementation. It wasn't noticed until now because the getVersion function isn't really used anywhere.

Test Plan: Ran arc linters in a repository with ArcanistJSONLintLinter configured, noticed that the version number appeared in the output.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T4954

Differential Revision: https://secure.phabricator.com/D9160