Preconfigured Grafana instance in Kubernetes link broken in README.md

[stenreijers]

The link to https://github.com/pixie-labs/pixielabs/tree/main/k8s/grafana_demo in the README.md is broken.

[kpattaswamy]

Hi @stenreijers, this link should work for now: https://github.com/pixie-io/pixie/tree/main/k8s/grafana_demo