Doc nginx passthrough secret as argocd-server-tls

As documented in tls.md § "TLS certificates used by argocd-server", the secret that should be modified by cert-manager is not `argocd-server` but `argocd-server-tls` should be created instead.

The ingress-nginx controller doesn't like using a secret of `type: Opaque`.

Signed-off-by: Josh Gardner <111776680+jdg-journeyfront@users.noreply.github.com>

docs/operator-manual/ingress.md

@@ -240,7 +240,7 @@ spec:
   tls:
   - hosts:
     - argocd.example.com
-    secretName: argocd-secret # do not change, this is provided by Argo CD
+    secretName: argocd-server-tls # as expected by argocd-server
 ```
 
 ### Option 2: Multiple Ingress Objects And Hosts