Update featuredImage property to include .node

Update accessing featuredImage sourceUrl to include .node as part of the
upgrade of WPGraphQL to 0.12.1

Before:
featuredImage.sourceUrl

After:
featuredImage.node.sourceUrl

See #17

Author: salcode

components/cover-image.js

@@ -5,7 +5,7 @@ import Link from 'next/link'
 export default function CoverImage({title, coverImage, slug}) {
   const image = (
     <img
-      src={coverImage?.sourceUrl}
+      src={coverImage?.node?.sourceUrl}
       className={cn('shadow-small', {
         'hover:shadow-medium transition-shadow duration-200': slug
       })}

pages/posts/[slug].js

@@ -37,7 +37,7 @@ export default function Post({post, posts, preview}) {
                 </title>
                 <meta
                   property="og:image"
-                  content={post.featuredImage?.sourceUrl}
+                  content={post.featuredImage?.node?.sourceUrl}
                 />
               </Head>
               <PostHeader