Version selector doesn't appear on Doxygen pages

[rcurtin]

There's supposed to be a version selector in the upper right hand corner on pages like this:

https://www.mlpack.org/doc/mlpack-3.2.2/doxygen/index.html

but it doesn't appear to be displaying. It's likely a simple fix; perhaps some JS isn't loading correctly, or something. I haven't even looked at the console when loading the page yet; I just wanted to write this down so it doesn't get forgotten. :)