Fix inexistent layout warning for the default 404 page

Warning message:
```
Layout 'default' requested in 404.html does not exist.
```

Probably introduced by the Minima v3 upgrade:
https://github.com/jekyll/minima?tab=readme-ov-file#base-layout

lib/site_template/404.html

@@ -1,6 +1,6 @@
 ---
 permalink: /404.html
-layout: default
+layout: page
 ---
 
 <style type="text/css" media="screen">