Fix a typo.

Summary: The lessc binary is a part of the NPM less module.

Test Plan: npm install less works whereas npm install lessc does not.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9006