nixos/nginx: fix comment about acme postRun not running as root

As of 67a5d66 this is no longer true, since acme postRun runs as root.
The idea of the service is good so reword a comment a bit.

nixos/modules/services/web-servers/nginx/default.nix

@@ -867,8 +867,9 @@ in
       source = configFile;
     };
 
-    # postRun hooks on cert renew can't be used to restart Nginx since renewal
-    # runs as the unprivileged acme user. sslTargets are added to wantedBy + before
+    # This service waits for all certificates to be available
+    # before reloading nginx configuration.
+    # sslTargets are added to wantedBy + before
     # which allows the acme-finished-$cert.target to signify the successful updating
     # of certs end-to-end.
     systemd.services.nginx-config-reload = let