fix: Apply consistent USWDS class in slim Footer storybook example

[brandonlenz]

Summary

There was some off wrapping behavior on this story that I noticed while testing #3266. The USWDS example was working fine. Turns out one of our classes in the story was simply (likely) copy pasted incorrectly at some point.

How To Test

yarn storybook

Then fiddle with screen width

Screenshots (optional)

Before:

After:

Author & Maintainer checklist

• Is this is a breaking change?
  • Yes, and I accounted for the breaking changes according to the linked documentation
  • No
• Once merged, add the PR and Issue author(s) as a contributor(s) via the all-contributors bot

[brandonlenz]

The context of why this change needed to be made is more visible expanding the section above to see the other grid col class:

[brandonlenz]

When grid col's don't match, things get weird 😆

[rswerve]

makes sense!