Update cert-manager API version in README

[irbekrm]

v1alpha2 cert-manager API referred to in the README is getting deprecated.

Possible Solution

Update the example configs to use the latest v1 API.

[alexellis]

Hi @irbekrm thank you for your interest in the project.

Whilst we appreciate the issue, you do need to fill out every field of the issue template. They are not optional.

https://raw.githubusercontent.com/openfaas/ingress-operator/master/.github/ISSUE_TEMPLATE.md

Thanks again.

Alex