Fix reactiflux url (#30709)

Summary:
On support page [https://github.com/facebook/react-native/blob/master/.github/SUPPORT.md](url) there's an url to reactiflux with missing '.com' suffix :-)

## Changelog

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[GENERAL] [Fixed] - Updated reactiflux url

Pull Request resolved: https://github.com/facebook/react-native/pull/30709

Test Plan: No test needed

Reviewed By: kacieb

Differential Revision: D25916668

Pulled By: TheSavior

fbshipit-source-id: a6193ec67bd8faf64a581036a891d1f9aeb1fd1d