[landing] [fix] button header on mobile breaking layout

Summary:
fix header so button in header doesn't show on mobile

before:

after:

(side stepping normal PR review because we have a lot of folks looking at the webpage this week.) I felt this was urgent to fix.

Test Plan: shrink viewport to show break point is working as expected and layout isn't broken

Reviewers: atul, ashoat

Reviewed By: ashoat

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

Differential Revision: https://phabricator.ashoat.com/D3231