Fix to improve example

Closes GH-1961.

Reviewed-by: Titus Wormer <tituswormer@gmail.com>

Author: Dawid Janiga

docs/guides/frontmatter.server.mdx

@@ -89,7 +89,7 @@ Like so:
 title: Hi, World!
 ---
 
-# {frontmatter.title}
+# {title}
 ```
 
 That’s exactly what the remark plugin