fix toc wrong typing

contentlayer.config.ts

@@ -54,7 +54,7 @@ const computedFields: ComputedFields = {
     type: 'string',
     resolve: (doc) => doc._raw.sourceFilePath,
   },
-  toc: { type: 'string', resolve: (doc) => extractTocHeadings(doc.body.raw) },
+  toc: { type: 'json', resolve: (doc) => extractTocHeadings(doc.body.raw) },
 }
 
 /**