Fix: Contentful webhook body parse. (#40732)

## Bug

- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have a 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
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have a helpful link attached, see `contributing.md`

## Documentation / Examples

- [ ] Make sure the linting passes by running `pnpm lint`
- [ ] The "examples guidelines" are followed from [our contributing doc](https://github.com/vercel/next.js/blob/canary/contributing/examples/adding-examples.md)

examples/cms-contentful/pages/api/revalidate.js

@@ -14,7 +14,7 @@ export default async function handler(req, res) {
   }
 
   try {
-    let postSlug = req.body.fields.slug['en-US']
+    let postSlug = JSON.parse(req.body).fields.slug['en-US']
 
     // revalidate the individual post and the home page
     await res.revalidate(`/posts/${postSlug}`)