[landing] Fix "fluid" font-size for hero-content:sub_heading

Summary: Continues to look as expected. Maintains proportion with the cycling header now which seems like it's actually a slight improvement.

Test Plan: Here's how it looks:

Reviewers: benschac, palys-swm, ashoat

Reviewed By: ashoat

Subscribers: Adrian, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3435