Update Prometheus screenshot, update helm repo url

[mszostok]

Description

Changes proposed in this pull request:

• Update Prometheus screenshot after https://github.com/kubeshop/botkube/pull/1005,
• Update Helm repo URL as reported here: https://github.com/kubeshop/botkube/issues/987

Testing

• https://update-prom-and-url.botkube-docs.pages.dev/configuration/executor/helm
• https://update-prom-and-url.botkube-docs.pages.dev/next/usage/source/prometheus

Related issue(s)

• https://github.com/kubeshop/botkube/pull/1005
• https://github.com/kubeshop/botkube/issues/987