Linkerd-viz: Update grafana setting when using 2.12.x

https://github.com/linkerd/website/blob/main/linkerd.io/content/2.12/tasks/grafana.md?plain=1#L53-L74
The setting for grafana should be
```yaml
grafana.ini:
server:
root_url: '%(protocol)s://%(domain)s:%(http_port)s/grafana/'
serve_from_sub_path: true
```

otherwise if not using port
```yaml
grafana.ini:
server:
root_url: '%(protocol)s://%(domain)s:/grafana/'
serve_from_sub_path: true
```
so the `grafana.url` should be below
```bash
linkerd viz install --set grafana.url=grafana.grafana \
| kubectl apply -f -
```
doesn't need port

Open linkerd.io/content/2.12/tasks/grafana.md around the referenced lines and review the Grafana configuration and linkerd viz install command. Update the documented root URL and grafana.url examples to match the requested port behavior, then verify the rendered instructions remain consistent.

Written by the indexing model from the issue text.