update readme

README.md

@@ -33,7 +33,7 @@ However the *CERT_PASSWORD* argument is required, as it's necesarry in order to
 You can build the image yourself like this
 
 ```sh
-docker build --build-arg supersecretpassword . -t net-base
+docker build --build-arg CERT_PASSWORD=supersecretpassword . -t net-base
 ```
 
 | Argument                                            | Description                                                             | Default                                                                                      |