fix nav borders

[reigj1]

Thank you for your contribution to the IC Developer Portal. This repo contains the content for https://internetcomputer.org and the ICP Developer Documentation, https://internetcomputer.org/docs/.

If you are submitting a Pull Request for adding or changing content on the ICP Developer Documentation, please make sure that your contribution meets the following requirements:

• Follows the developer docs style guide.
• Follows the best practices and guidelines.
• New documentation pages include document tags.
• New documentation pages include SEO keywords.
• New documents are in the .mdx file format to support the previous two components.
• New documents are registered in /sidebars.js, otherwise, it will not appear in the
  side navigation bar.
• Make sure that the .github/CODEOWNERS file is
  filled with new documents that you added. This way we can ensure that future Pull Requests are reviewed by the right people.