Fix typo in rewrites docs (vercel#35639)

Correct the type of `trailingSlash` in config example

Modified: `docs/api-reference/next.config.js/rewrites.md`

```diff
- trailingSlash: 'true',
+ trailingSlash: true,
```

docs/api-reference/next.config.js/rewrites.md

@@ -327,7 +327,7 @@ If you're using `trailingSlash: true`, you also need to insert a trailing slash
 
 ```js
 module.exports = {
-  trailingSlash: 'true',
+  trailingSlash: true,
   async rewrites() {
     return [
       {