Fix: `SubscriptionButton` format

[mariana-caetano]

Describe the bug
Fix the button from the SubscriptionList component to be in the same format as the Help Center example (PR):

Expected behavior
The button should follow a rectangle format and be aligned with the input container.

Screenshots
If applicable, add screenshots to help explain your problem.

Desktop (please complete the following information):

• OS: [e.g. iOS]
• Browser [e.g. chrome, safari]
• Version [e.g. 22]

Smartphone (please complete the following information):

• Device: [e.g. iPhone6]
• OS: [e.g. iOS8.1]
• Browser [e.g. stock browser, safari]
• Version [e.g. 22]

Additional context
Add any other context about the problem here.

[mariana-caetano]

Task: EDU-17484