Use "https://" instead of "git://" for anoymous GitHub requests in documentation

Summary: These both work, but "git://" uses a nonstandard and possibly firewalled port, is less familiar to users, and is not promoted in the GitHub UI.

Test Plan: grep, reviewed diff.

Reviewers: chad, avive, avivey

Reviewed By: avivey

Subscribers: epriestley, avivey

Differential Revision: https://secure.phabricator.com/D9360