[landing] fix size prop for icons in mobile nav

Summary:
As I was debugging D8175, I noticed in the console that I was getting error logs because I accidentally passed the wrong size prop into the icons in the mobile nav component. This diff fixes that issue

Depends on D8175

Test Plan: The error message is now gone, and the icons are still the same size

Reviewers: atul, kamil

Reviewed By: atul

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8176