Add module.exports to security headers documentation (#26466)

Without `module.exports`, the provided code won't work if just pasted into `next.config.js`

## Documentation / Examples

- [x] Make sure the linting passes

Author: samrobbins85

docs/advanced-features/security-headers.md

@@ -11,16 +11,18 @@ To improve the security of your application, you can use [`headers`](/docs/api-r
 
 // You can choose which headers to add to the list
 // after learning more below.
-const securityHeaders = [];
-
-async headers() {
-  return [
-    {
-      // Apply these headers to all routes in your application.
-      source: '/(.*)',
-      headers: securityHeaders
-    }
-  ]
+const securityHeaders = []
+
+module.exports = {
+  async headers() {
+    return [
+      {
+        // Apply these headers to all routes in your application.
+        source: '/(.*)',
+        headers: securityHeaders,
+      },
+    ]
+  },
 }
 ```