Add margin between buttons on backup brave wallet dialog

[luixxiul]

Fix #8736

Auditors:

Test Plan:

1. Open about:preferences#payments
2. Open advanced settings
3. Click "Back up your wallet"

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).

[cezaraugusto]

++ thanks for taking that