readme: ui: use --guestbook-ca="" as we now use a lets encrypt cert

Signed-off-by: Maël Valais <[email protected]>

README.md

@@ -676,17 +676,18 @@ Now, ssh into the Raspberry Pi and launch the UI:
 ```sh
 # From your laptop.
 ssh pi docker rm -f print-your-cert-ui
-ssh pi 'kubectl get secret -n cert-manager root-print-your-cert-ca --template="{{index .data \"tls.crt\" | base64decode}}" | tee ca.crt'
 ssh pi docker run -d --restart=always --name print-your-cert-ui --net=host \
   -v '/home/pi/.kube/config:/home/nonroot/.kube/config' \
-  -v '/home/pi/ca.crt:/home/nonroot/ca.crt' \
   ghcr.io/cert-manager/print-your-cert-ui:latest \
   --issuer print-your-cert-ca \
   --issuer-kind ClusterIssuer \
   --listen 0.0.0.0:8080 \
-  --guestbook-ca /home/nonroot/ca.crt
+  --guestbook-ca=""
 ```
 
+Set `--guestbook-ca` to `""` because we manually issued a Let's Encrypt
+certificate for the guestbook.
+
 > [!NOTE]
 >
 > We don't actually push the image to GHCR. We just load it directly to the Raspberry Pi.