Fix preview comment showing .json source paths instead of .html page URLs

The link text for single-file entries used the source filename (e.g.
`epub.json`) instead of the rendered page path (`epub.html`), making
the comment inconsistent with multi-file entries which already showed
the page path.

(cherry picked from commit a08bd51)

Author: cderv

.github/workflows/preview.yml

@@ -211,7 +211,7 @@ jobs:
                 for (const [page, files] of pageToFiles) {
                   const fileUrl = `${deployUrl}/${page}`;
                   if (files.length === 1) {
-                    commentBody += `- [${files[0]}](${fileUrl})\n`;
+                    commentBody += `- [${page}](${fileUrl})\n`;
                   } else {
                     // Multiple source files map to one page - show page with file summary
                     const basenames = files.map(f => f.split('/').pop());