Fix the proposed noise private_key_path

As indicated by the comment, the default /var/lib/headscale path is not writable in the container. However the sample setting is not following that like `private_key_path`

docs/running-headscale-container.md

@@ -55,7 +55,7 @@ metrics_listen_addr: 0.0.0.0:9090
 private_key_path: /etc/headscale/private.key
 # The default /var/lib/headscale path is not writable in the container
 noise:
-  private_key_path: /var/lib/headscale/noise_private.key
+  private_key_path: /etc/headscale/noise_private.key
 # The default /var/lib/headscale path is not writable  in the container
 db_path: /etc/headscale/db.sqlite
 ```