[native] Change 'Create Community' to 'Create community'

Summary: Requested in D8379, this just changes the button copy to match the capitalization across the app

Test Plan:
Checked that button updated (screenshot below)

Reviewers: atul, ashoat, ginsu

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D8427