Update flag name in private_network_docs

Signed-off-by: Lee Smet <lee.smet@hotmail.com>

docs/private_network.md

@@ -28,8 +28,8 @@ Not all cipher suites supported in TLS1.3 are supported. At present, _at least_
 ## Enable private network
 
 If you are running the standard `mycelium` binary, private network functionality
-can be enabled by setting both the `network-name` and `network-key` flags on the
-command line. All nodes who which to join the network must use the same values
+can be enabled by setting both the `network-name` and `network-key-file` flags on
+the command line. All nodes who which to join the network must use the same values
 for both flags.
 
 > ⚠️ Network name is public, do not put any confidential data here.