[2/2] [landing] style hero cta button per figma

Summary: style hero cta button per figma

Test Plan: view the app on a mobile device and desktop

Reviewers: atul, ashoat

Reviewed By: atul, ashoat

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

Differential Revision: https://phabricator.ashoat.com/D2637