07-flex-layout-2: Update README.md for consistency with solution.css

[zhangsherry]

Because

Since commit 65b831b changed the padding of cards container in solution.css, the instruction of padding in README.md should be consistent.

This PR

• Update the padding of 'cards' section from 32px to 48px.

Issue

Closes #XXXXX

Additional Information

Although commit 65b831b changed margin size of every card as well, it's fine because there is nothing about margin size of cards in README.md.

Pull Request Requirements

• [x] I have thoroughly read and understand The Odin Project Contributing Guide
• [x] The title of this PR follows the location of change: brief description of change format, e.g. 01-flex-center: Update self check
• [x] The Because section summarizes the reason for this PR
• [x] The This PR section has a bullet point list describing the changes in this PR
• [x] If this PR addresses an open issue, it is linked in the Issue section
• [x] If applicable, I have ensured that the TOP solution files match the Desired Outcome image

[MaoShizhong]

Great catch, thank you for PRing this.