fix: add a slash into the canonical URL

Co-authored-by: Lemin-n <[email protected]>

Author: Phosphorus-M

src/renderer/html_handlebars/hbs_renderer.rs

@@ -304,7 +304,7 @@ impl HtmlHandlebars {
             ch.name.clone() + " - " + book_title
         };
 
-        let slug = ctx_path.replace(".md", "").replace("/index", "");
+        let slug = ctx_path.replace(".md", "").replace("/index", "/");
 
         let mut site_url = if let Some(site_url) = &ctx.html_config.site_url {
             site_url.clone()