fix: wiggling nav items

[huyenltnguyen]

Checklist:

• [x] I have read freeCodeCamp's contribution guidelines.
• [x] My pull request has a descriptive title (not a vague title like Update index.md)

This PR changes the box model from content-box to border-box to prevent the nav items from wiggling on hover.

Screenshots (containing animated images)

| Before | After | | --- | --- | |  |  |

[raisedadead]

Please merge and deploy when ready.