update banner example

Author: dks333

settings/global.mdx

@@ -422,8 +422,10 @@ Every documentation site requires a `docs.json` file that contains the core conf
   <Expandable title="Banner">
     <ResponseField name="content" type="string">
       The content of the banner. This can be a string of text or a markdown string. For example:
-      ```mdx
-      🚀 Banner is live! [Learn more](mintlify.com)
+      ```json
+      {
+        "content": "🚀 Banner is live! [Learn more](mintlify.com)"
+      }
       ```
     </ResponseField>
     <ResponseField name="dismissible" type="boolean">