Stop advising users to use npm -g to install websockets

Summary: Fixes T6910. This advice is bad, doesn't work, and was based on me havng an outdated or incorrect understanding of Node and npm.

Test Plan: Read documentation.

Reviewers: richardvanvelzen, btrahan, chad, joshuaspence

Reviewed By: chad, joshuaspence

Subscribers: epriestley

Maniphest Tasks: T6910

Differential Revision: https://secure.phabricator.com/D11285