fix: front-matter prop change

src/content/posts/markdown.md

@@ -2,9 +2,7 @@
 title: Markdown Example
 published: 2023-10-01
 description: A simple example of a Markdown blog post.
-cover:
-  url:
-  alt:
+image:
 tags: [Markdown, Blogging, Demo]
 categories: [Example]
 ---