acme-client.conf example: more explicit clue to test with staging server

[Theo Buehler]

espie mentioned that the clue to use the staging server could be more explicit.
Maybe this is enough and not too intrusive?

Index: examples/acme-client.conf
===================================================================
RCS file: /cvs/src/etc/examples/acme-client.conf,v
retrieving revision 1.4
diff -u -p -r1.4 acme-client.conf
--- examples/acme-client.conf   17 Sep 2020 09:13:06 -0000      1.4
+++ examples/acme-client.conf   9 May 2023 18:46:15 -0000
@@ -27,5 +27,7 @@ domain example.com {
        alternative names { secure.example.com }
        domain key "/etc/ssl/private/example.com.key"
        domain full chain certificate "/etc/ssl/example.com.fullchain.pem"
-       sign with letsencrypt
+       # Test with the staging server to avoid aggressive rate-limiting.
+       #sign with letsencrypt
+       sign with letsencrypt-staging
 }