Update webhook troubleshooting documentation to including necessary curl command.

[firstmustburn]

The documentation has been update to reflect a needed change in the curl incantation for the webhook troubleshooting.

Using the original listed curl command to check the webhook (after setting up the forward):

curl -vsS --resolve cert-manager-webhook.cert-manager.svc:10250:127.0.0.1 \
    --service-name cert-manager-webhook-ca \
    --cacert <(kubectl -n cert-manager get secret cert-manager-webhook-ca -ojsonpath='{.data.ca\.crt}' | base64 -d) \
    https://cert-manager-webhook.cert-manager.svc:10250/validate 2>&1 -d@- <<'EOF' | sed '/^* /d; /bytes data]$/d; s/> //; s/< //'
{"kind":"AdmissionReview","apiVersion":"admission.k8s.io/v1","request":{"requestKind":{"group":"cert-manager.io","version":"v1","kind":"Certificate"},"requestResource":{"group":"cert-manager.io","version":"v1","resource":"certificates"},"name":"foo","namespace":"default","operation":"CREATE","object":{"apiVersion":"cert-manager.io/v1","kind":"Certificate","spec":{"dnsNames":["foo"],"issuerRef":{"group":"cert-manager.io","kind":"Issuer","name":"letsencrypt"},"secretName":"foo","usages":["digital signature"]}}}}
EOF

I get back

{"response":{"uid":"","allowed":false,"status":{"metadata":{},"message":"contentType=application/x-www-form-urlencoded, expected application/json","code":400}}}

Then I add -H "Content-Type: application/json" :

curl -vsS --resolve cert-manager-webhook.cert-manager.svc:10250:127.0.0.1 \
    --service-name cert-manager-webhook-ca \
    -H "Content-Type: application/json" \
    --cacert <(kubectl -n cert-manager get secret cert-manager-webhook-ca -ojsonpath='{.data.ca\.crt}' | base64 -d) \
    https://cert-manager-webhook.cert-manager.svc:10250/validate 2>&1 -d@- <<'EOF' | sed '/^* /d; /bytes data]$/d; s/> //; s/< //'
{"kind":"AdmissionReview","apiVersion":"admission.k8s.io/v1","request":{"requestKind":{"group":"cert-manager.io","version":"v1","kind":"Certificate"},"requestResource":{"group":"cert-manager.io","version":"v1","resource":"certificates"},"name":"foo","namespace":"default","operation":"CREATE","object":{"apiVersion":"cert-manager.io/v1","kind":"Certificate","spec":{"dnsNames":["foo"],"issuerRef":{"group":"cert-manager.io","kind":"Issuer","name":"letsencrypt"},"secretName":"foo","usages":["digital signature"]}}}}
EOF

And I get back the expected response:

{"kind":"AdmissionReview","apiVersion":"admission.k8s.io/v1","response":{"uid":"","allowed":true,"status":{"metadata":{},"code":200}}}
I think getting the other answer could be confusing to people using the guide which is so excellent and detailed.

[cert-manager-prow]

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[ThatsMrTalbot]

Nit: can you start the sentence with a capital letter

Suggested change

	> ℹ️ if the `-H "Content-Type: application/json"` is omitted from the command above, you are likely
	> ℹ️ If the `-H "Content-Type: application/json"` is omitted from the command above, you are likely