update doc regarding health check

[sdarwin]

Hi,
A proposal to remove a "caveat" from the page.
Here are some different supporting factors:

• The warning is 3 years old.
• The issue it references has been closed. examples/k8s: no way to query healthz endpoint if dex is running on a custom CA #682.
• I've tested an instance of k8s dex with TLS & health checks. It worked.
• The project dex-k8s-authenticator has an example dex chart with TLS & health checks. That's been available for some time.
• When k8s does a probe: "If scheme field is set to HTTPS, the kubelet sends an HTTPS request skipping the certificate verification." Since certificate verification is skipped, there shouldn't be a problem with a custom CA.

[bonifaido]

You're right, Thanks!