[native] fix no spacing in the community creation add members screen

Summary:
The add members section of the community creation add members screen didn't have any spacing. This diff fixes that

Linear task: https://linear.app/comm/issue/ENG-6011/add-minimal-margin-in-the-add-members-screen-in-community-creation

Test Plan:
Please see the screenshot below

before:

after:

Reviewers: atul, inka

Reviewed By: atul

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10988