fix: choosing featured image in Netlify CMS

Fixes conflict between `featured` param describing if a post is featured and featured `image` param - was a typo with it also being named `featured` instead of the unique name of `image`.

Fix https://github.com/gcushen/hugo-academic/issues/1593

static/admin/config.yml

@@ -42,7 +42,7 @@ collections:
         required: false
         widget: "list"
       - label: "Featured Image"
-        name: "featured"
+        name: "image"
         widget: object
         fields:
           - label: "Upload an image named `featured.jpg/png`"