Update README.md

[fiskhest]

Noticed this didn't match with the repositories path.

❯ helm repo add silverbullet https://bnason.github.io/silverbullet-helm
Error: looks like "https://bnason.github.io/silverbullet-helm" is not a valid chart repository or cannot be reached: failed to fetch https://bnason.github.io/silverbullet-helm/index.yaml : 404 Not Found

The updated path works:

❯ helm repo add silverbullet https://bnason.github.io/silverbullet-chart
"silverbullet" has been added to your repositories