Fix Broken Images in Primer Theme

[rickstaa]

I've noticed that two images in the Primer theme are currently not displaying correctly:

1. Primer thumbnail image at https://github.com/pages-themes/primer:

2. Documentation large image at https://pages-themes.github.io/primer/:

Pull Request #62 was initiated to address a similar issue for the README image, but the problem persists for the images mentioned above.

To resolve this, I have submitted Pull Request #86, which aims to fix the display issue for the large image on the GitHub Pages example site.

Please review and merge the changes if they meet the project's requirements. This should help ensure that all visuals in the Primer theme are displayed correctly.