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