Update contributors page

[koji-1009]

fix #61

[github-actions[bot]]

Visit the preview URL for this PR (updated for commit 0d90f18):

https://flutterkakgi-2023-conf-app-dev--pr96-refactor-contribu-n8uj8zel.web.app

_{(expires Sat, 09 Sep 2023 04:01:25 GMT)}

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_{Sign: 1cac8c9fbcf8c2f43fea34942176b864ab92a752}

[koji-1009]

@FlutterKaigi/conference-app-2023 レビューをお願いします。