Update firewall creation command in README (#258)

Use correct firewall create instead of machine create.

Also use same placeholder for the network as in the previous machine create command to avoid confusion

Author: dhilgarth

README.md

@@ -137,15 +137,15 @@ docker compose run --rm metalctl machine create \
         --networks <network-ID>
 
 # create a firewall that is also connected to the virtual internet-mini-lab network
-docker compose run --rm metalctl machine create \
+docker compose run --rm metalctl firewall create \
         --description fw \
         --name fw \
         --hostname fw \
         --project 00000000-0000-0000-0000-000000000000 \
         --partition mini-lab \
         --image firewall-ubuntu-3.0 \
         --size v1-small-x86 \
-        --networks internet-mini-lab,$(privatenet)
+        --networks internet-mini-lab,<network-ID>
 ```
 
 See the installation process in action