[native] [5/34] ReactNav6: Type getComponent prop of ScreenComponent

Summary:
This actually doesn't appear to be new in ReactNav6. I noticed the libdef was incorrect when reviewing the docs.

Depends on D5397

Test Plan: Flow + ran the app on my phone and played around with it

Reviewers: atul, tomek, bartek

Reviewed By: atul

Subscribers: abosh

Differential Revision: https://phab.comm.dev/D5398