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