Fixed Mention Admin API mapper

There was a typo when reading the featured image property causing it not to show
up in the API responses.

ghost/core/core/server/api/endpoints/utils/serializers/output/mappers/mentions.js

@@ -13,6 +13,6 @@ module.exports = (model) => {
         source_excerpt: json.sourceExcerpt,
         source_author: json.sourceAuthor,
         source_favicon: json.sourceFavicon,
-        source_featured_image: json.sourceFeauredImage
+        source_featured_image: json.sourceFeaturedImage
     };
 };