[FedCM] Increase bottom margin for multi IDP

The current margin still creates some jag, so increase further, and
match the widget mode margins.

Bug: 325503352
Change-Id: Ida78e70e0b114f58815b858f2668c55a6064d113
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5714334
Reviewed-by: Zachary Tan <tanzachary@chromium.org>
Commit-Queue: Zachary Tan <tanzachary@chromium.org>
Auto-Submit: Nicolás Peña <npm@chromium.org>
Commit-Queue: Nicolás Peña <npm@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1328419}