fix broken link on RSS feeds

On the top "subscribe" button, the link has two
double-dashes. Surprisingly, this doesn't work.

For example, hosting our site on grebedoc.dev led to this link
becoming a 404, and I suspect that's the page on any hosting provider
behind a "git-pages" implementation. We can also reproduce this on our
legacy Apache web servers.

We already require the base URL to have a trailing slash elsewhere, so
let's not add a double here.

This is tracked upstream as issue 357 and PR 358.