Enclose secret in single quotes (#399)

Author: kbatuigas

modules/manage/pages/cluster-maintenance/config-cluster.adoc

@@ -45,7 +45,7 @@ To set a cluster property with a secret, you must use the following notation:
 
 [source,bash]
 ----
-rpk cluster config set iceberg_rest_catalog_client_secret ${secrets.<secret-name>}
+rpk cluster config set iceberg_rest_catalog_client_secret '${secrets.<secret-name>}'
 ----
 
 NOTE: Some properties require a rolling restart, and it can take several minutes for the update to complete. The `rpk cluster config set` command returns the operation ID.