fix links in README
fix links in README

Summary:
master -> main, made twitter badge consistent

Changelog: [internal]

Reviewed By: bradzacher

Differential Revision: D32466437

fbshipit-source-id: 5bc88bb2cdb178b08f6314d5c8b28e2e7767569a

mroch/flow,facebook/flow,mroch/flow,facebook/flow,facebook/flow,mroch/flow,facebook/flow,mroch/flow,mroch/flow,facebook/flow,mroch/flow,facebook/flow,mroch/flow,facebook/flow