[DOCS] Update variance.md
[DOCS] Update variance.md

Summary:
Saw minor typo while reading documentation

Pull Request resolved: https://github.com/facebook/flow/pull/8566

Reviewed By: panagosg7

Differential Revision: D26893503

Pulled By: nmote

fbshipit-source-id: 8b4d4e05137cd9b812a0ecc33702a08fc53c5f14

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