Fix footer link for Plain Writing

[greg-pf]

🎫 Ticket

https://jira.cms.gov/browse/BCDA-xxx

🛠 Changes

Fixed a broken link in the footer to the plain writing guide.

ℹ️ Context for reviewers

Clicking the "Plain Writing" link in the footer, brings the user to a 404 page

✅ Acceptance Validation

The updated link directs to

🔒 Security Implications

• This PR adds a new software dependency or dependencies.
• This PR modifies or invalidates one or more of our security controls.
• This PR stores or transmits data that was not stored or transmitted before.
• This PR requires additional review of its security implications for other reasons.

If any security implications apply, add Jason Ashbaugh (GitHub username: StewGoin) as a reviewer and do not merge this PR without his approval.

[greg-pf]

I clicked through the other links in the footer as well. All land on the expected pages.

[kyeah]

⭐