DOC: fix the Markdown example for static index page URL override

Probably the smallest pull request ever.  I found that if, in Markdown, URL is set to `/`, the links are corrupted (I think they end up blank).  By trial and error I found that the empty string properly points to the site url. It seems an extra slash is prepended to what the user sets.  If URL is `/`, the link ends up as `<a href="//" ...` which is broken.