docs: adding changes to setCookie example (vercel#36870)

fixes vercel#28453

## Bug

- [x] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have helpful link attached, see `contributing.md`

## Feature

- [ ] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [X] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have helpful link attached, see `contributing.md`

## Documentation / Examples

- [X] Make sure the linting passes by running `yarn lint`

docs/api-routes/api-middlewares.md

@@ -165,9 +165,8 @@ export const setCookie = (
   const stringValue =
     typeof value === 'object' ? 'j:' + JSON.stringify(value) : String(value)
 
-  if ('maxAge' in options) {
-    options.expires = new Date(Date.now() + options.maxAge)
-    options.maxAge /= 1000
+  if (typeof options.maxAge === 'number') {
+    options.expires = new Date(Date.now() + options.maxAge * 1000)
   }
 
   res.setHeader('Set-Cookie', serialize(name, stringValue, options))