Update AppVeyor badge in README.md
Update AppVeyor badge in README.md

Summary:
Badge was out of date
Closes https://github.com/facebook/flow/pull/2073

Differential Revision: D3552148

Pulled By: gabelevi

fbshipit-source-id: b22f96c6ed0d65a5fb80c57e30ee8af7b4972eea

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