docs: update code block filename in dynamic API context error page (#67000)

The filenames of the two code blocks on this page
https://nextjs.org/docs/messages/next-dynamic-api-wrong-context are
written as comment instead of using the `filename` prop.


![image](https://github.com/vercel/next.js/assets/20135478/86b45742-cb83-4cd1-933c-a1c22f51e0fd)

---------

Signed-off-by: Eng Zer Jun <engzerjun@gmail.com>
Co-authored-by: Sam Ko <sam@vercel.com>

errors/next-dynamic-api-wrong-context.mdx

@@ -14,19 +14,17 @@ Make sure that all Dynamic API calls happen in a request scope.
 
 Example:
 
-```diff
-// app/page.ts
+```jsx filename="app/page.js"
 import { cookies } from 'next/headers'
 
-- const cookieStore  = cookies()
+- const cookieStore = cookies()
 export default function Page() {
 + const cookieStore = cookies()
   return ...
 }
 ```
 
-```diff
-// app/foo/route.ts
+```jsx filename="app/foo/route.js"
 import { headers } from 'next/headers'
 
 - const headersList = headers()