callouts: emit type as string attribute

lib/callout-directive.js

@@ -26,7 +26,7 @@ export default function calloutDirective() {
         attrs.push({
           type: 'mdxJsxAttribute',
           name: 'type',
-          value: { type: 'mdxFlowExpression', value: `"${name}"` },
+          value: name,
         });
 
         // Build children from node's children