[Docs] Use sentence case instead of title case

Summary:
I previously used title case, however, the rest of the developer
documenation uses sentence case. Just make it more consistent.

Depends on D4965

Test Plan: N/A

Reviewers: ashoat, varun

Reviewed By: ashoat

Subscribers: tomek, atul, abosh

Differential Revision: https://phab.comm.dev/D4966