Fix markup in documentation for arcanist lint engine

Summary: Fixed wrong markup which was 1 space instead of 2

Test Plan: I couldn't test it as I don't know how to build documentation

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D7587