Dead link to libSBML Rsource package.

[LaneMatthewJ]

The bug

In the installation documentation, there exists a supposed link to the libSBML R package. When clicking on the link, a 404 occurs.

Upon digging through the code the issue appears to be here:
https://github.com/sbmlteam/libsbml/blob/49ce5b40ead6862c1a610a569d0606f03a4fb4fa/docs/src/common-text/libsbml-downloading.html.in#L158-L162

To Reproduce

Click the link here

[fbergmann]

This issue is with the Hugo download page. So I transferred it here to sort this.