Fixed incorrect helm template syntax in README.md

[kaushall]

Helm uses {{ }} for its own templating, which can conflict with the Go templating syntax used in OAuth2 Proxy's templates. This change will make it working.

[pierluigilenoci]

@kaushall, thank you for your contribution.
To proceed with the approval of the PR, we would like to ask for a few additional adjustments.

• Please update the Helm chart version https://github.com/oauth2-proxy/manifests/blob/main/helm/oauth2-proxy/Chart.yaml#L2
• Revise the notes for ArtifactHub https://github.com/oauth2-proxy/manifests/blob/main/helm/oauth2-proxy/Chart.yaml#L36-L41

[kaushall]

Thanks @pierluigilenoci, I have made the changes.