fix(ras-acc): update shipping details header

[laurelfulford]

All Submissions:

• Have you followed the Newspack Contributing guideline?
• Does your code follow the WordPress' coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

This teeny PR switches the Shipping Details header from h2 to h3, both to match the hierarchy level of the Billing Details header, but also so it picks up the spacing styles.

How to test the changes in this Pull Request:

1. On epic/ras-acc, set up a product that requires shipping, assign it to a Checkout Button block, and run a checkout.
2. When you hit the second screen, note the spacing between the Billing Details heading and info, and the Shipping Details heading and info:

3. Apply this PR and run npm run build.
4. Run the checkout again and confirm that the spacing looks the same:

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?

[chickenn00dle]

Looks good!

[laurelfulford]

Thanks @chickenn00dle! 🙌