Update `cml-with-dvc` example.

Note that this hasn't been released yet but it's merged on master:

---

> > For the vega example in https://cml.dev/doc/cml-with-dvc using vl2png, how can this be handled with --json?
>
> Some json manipulation is needed for that. In this particular example:
>
> ```
> dvc plots diff \
> --target classes.csv \
> --template confusion \
> -x actual \
> -y predicted \
> --json master | jq '."classes.csv" | .[0]' >> vega.json
> ```

I think `send-comment` with vega plot is a widely used example

_Originally posted by @daavoo in https://github.com/iterative/dvc/issues/7093#issuecomment-998095565_