Update README.md

[tr-ace]

Fix link to roadmap

Description

Noticed the link to the roadmap.md file was broken, so I fixed it.

Related Issue

https://github.com/Vets-Who-Code/vets-who-code-app/issues/474

Motivation and Context

The roadmap is very important, and a broken link to it could cause new visitors frustration and lack of trust in the code.

How Has This Been Tested?

I changed the URL for the link in the README.md, and verified it takes me to roadmap.md when clicked.

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] My code follows the code style of this project.
• [x] My change requires a change to the documentation.
• [x] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed.

[vercel[bot]]

@tr-ace is attempting to deploy a commit to the vetswhocode-web-app Team on Vercel.

A member of the Team first needs to authorize it.

[github-actions[bot]]

Hey @tr-ace, thanks for opening your first Pull Request. This is an exciting time to contriubte to our open source appliucation. Keep up the great work! Someone will review it soon.