fix: showFullScreen should be camelcased (#866)

Author: dks333

image-embeds.mdx

@@ -102,7 +102,7 @@ For YouTube videos use:
   title="YouTube video player"
   frameborder="0"
   allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture"
-  allowfullscreen
+  allowFullScreen
 ></iframe>
 ```