STYLE: Fix incorrect path to ujson directory
- [x] closes #39347 

I fixed the path to `ujson` directory so `cpplint` will run on `pandas/_libs/src/ujson/` instead of `pandas/_libs/ujson`, which doesn't exist.

But the title of the issue says `cpplint (unintentionally?) skips pandas/_libs/src/ujson/`, so I understand there's a chance the `pandas/_libs/src/ujson/` is getting *intentionally* skipped.

Assuming this was **unintentional**, I believe this small fix solves the issue.