nixos/k3s: replace deprecated extra flag in usage example

The previously used `--kubelet-arg=v=4` extra flag causes k3s to crash
with `Error: initialize logging: the logging configuration should not be
changed after setting it once`. Replace it with the working `--debug`
flag.

pkgs/applications/networking/cluster/k3s/docs/USAGE.md

@@ -15,7 +15,7 @@
   services.k3s.enable = true;
   services.k3s.role = "server";
   services.k3s.extraFlags = toString [
-    # "--kubelet-arg=v=4" # Optionally add additional args to k3s
+    # "--debug" # Optionally add additional args to k3s
   ];
 }
 ```