In the docs, I'm seeing valid Markdown HTTP links rendering to HTML that Chrome interprets as HTTPS links.
The "semantic versioning" link on this page is a broken HTTPS link. The HTTP version works as expected (manual s/s/).
Viewing the page's source gives this HTML:
<a href="//semver.org/">semantic versioning</a>.
That link is generated by this Markdown:
[semantic versioning](http://semver.org/)
Note that the markdown uses an HTTP link and seems valid.
As another example, the "Ruby" link on this page is HTTPS according to Chrome.
It renders similar HTML:
<a href="//www.ruby-lang.org">Ruby</a>
It's generated by this Markdown:
[Ruby](http://www.ruby-lang.org)
Note that the Markdown link is also HTTP, not the HTTPS Chrome renders. However, this "Ruby" link isn't broken, as ruby-lang.org answers to HTTPS. The only reason the issue was noticeable for the semver link is because that site answers to HTTP only.
I'm honestly not sure where to put this bug. I see that Vagrant is using Middleman. Overall, it seems more likely that you've got something configured incorrectly or similar, vs a bug in Middleman. Not a web-dev expert, but glad to help as much as I can.