[web] Added extra margin in "Create subchannel" modal

Summary:
Changed margin in wrapper around Visibility Option Content in Create
Subchannel modal.

Test Plan:
Build app.
Visually checked if it looks correctly after changes.

Reviewers: tomek, kamil, atul, rohan, ginsu

Reviewed By: tomek, atul, ginsu

Subscribers: ashoat, abosh

Differential Revision: https://phab.comm.dev/D5314