[landing] [fix] [ENG-982] add correct type to button

Summary:
add the same onClick type that's currently in web/ project's button onClick handler.

https://github.com/CommE2E/comm/blob/master/web/components/button.react.js#L9

https://linear.app/comm/issue/ENG-982/landing-button-onclick-type-incorrect

Test Plan: use <Button></Button> component in landing page

Reviewers: atul, palys-swm

Reviewed By: palys-swm

Subscribers: ashoat, palys-swm, Adrian, karol-bisztyga, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3659